"I know Louis loves me, but sometimes I wonder if he understands me."
-Blair to Serena, Father and the Bride
Blair and Louis meet in Paris and go on a few dates before Blair returns to New York. Louis eventually goes to find her, and the two become engaged. However their relationship is strained, mainly due to Blair's unresolved feelings for Chuck Bass. At one point, Blair plans to leave Louis but ultimately doesn't. They marry, but their wedding is interrupted by a video of Blair's confession of love to Chuck, and Louis makes sure Blair knows he only married her to save face and behind the scenes, they have no relationship. Eventually, with the help of Georgina Sparks, Blair and Louis divorce.
Blair meets Louis in Belles de Jour as she's staring at a painting in an art museum in Paris. Louis introduces himself to her and tells her he's noticed her before and thinks she's beautiful. He also tells her he knows she's from New York as her watch is set to Eastern Standard Time. He asks her on a date, and she accepts. Before he leaves, a chauffeur tells him he's taking him to the embassy to meet with a Grimaldi. Blair thinks he may be a member of the the Grimaldi family, aka the royal family of Monaco. Louis says that he's just Louis and leaves. While shopping for Serena, Blair tells her she thinks Louis is a prince. Louis texts her and tells her their date is a double and Blair invites Serena to come along. When they're getting picked up, Louis says Serena's date is member of the royal family and he is not. Disappointed, Blair acts disinterested in the date the whole night. Afterwards, Louis reveals he is really the royal and it was a test to see if Blair was only after he title and she failed.
In Double Identity, Blair convinces Louis to give her another chance. They go on a date and she keeps it low-key, to prove she isn't only interested in his royalty lifestyle. He invites her to a ball, but they don't make it due to Blair going to save Chuck from running away from his life. The next day, she apologizes to him and he accepts. She tells him that she's heading back to New York and gives him her shoe, saying he's always welcome to come find her.
At the end of The Kids Stay in the Picture, Louis arrives in New York with Blair's shoe, as he's come to find her.
In Petty in Pink, Blair and Louis spend the day together in New York and try to stay inconspicuous, as no one can know he's there. Meanwhile, Dan Humphrey has been asked to write a story on Louis' trip in town but he quits when he finds out Louis is with Blair. When the Grimaldi's find out he's there, Blair devises a plan to convince his family to let him stay. The plan involves convincing everyone Louis is only there to attend the Pink Party and that she's really dating Dan. However, the plan doesn't quite work out. After the party, Louis asks Blair if she's fine with being public and the two kiss after exiting their limo, revealing their relationship to the world.
Louis' mother Sophie Grimaldi arrives in The Princesses and the Frog and she highly disapproves of Blair due to Blair's history of scandals and her commoner status. Serena doesn't help matters, as she gives Sophie a book full of Gossip Girl blasts about Blair's wrongdoings. Sophie informs Blair that Louis is to be married soon and is unavailable to her, however Louis explains that he's not engaged yet but getting pressure from his family. He tells Blair he convinced Sophie to give her another chance, and after sitting down with her and pleading her case, she agrees to let Blair come to a ball the next day. At the ball, Blair makes a good first impression on everyone until Chuck crashes and ruins the event. Blair leaves, and confronts Serena about giving Sophie the book, and she explains that she's ready to grow up and that she really did want Louis. Louis overhears and afterwards, proposes to Blair, who accepts.
Eleanor Waldorf and Cyrus Rose come back to New York in Shattered Bass to throw Blair and Louis an engagement party. At the party, Cyrus wins Sophie over, who agrees to let Blair and Louis make their first official appearance as an engaged couple that evening. Blair is over the moon, until Louis makes an ultimatum that he only will if Blair is willing to share all the parts of herself. He tells her that she has until the Constance/St. Jude party later to decide and if she doesn't, he will assume she won't make it work with him and leave. However, due to Chuck's dispute with Russell Thorpe, Blair is lured to The Empire where she is held captive until Chuck saves her in The Wrong Goodbye. Afterwards, the two have sex and Blair is ready to leave Louis. She mentions to Chuck that while Louis makes her happy, he is not her true love. She goes to meet Louis, who is still waiting for her at Constance despite her taking so long. Before she can break it off with Louis, Chuck jumps in and gives his blessing. Because of this, Blair is with Louis. In the time jump a few weeks later, it's revealed that Blair is planning to spend the whole summer with Louis in Monaco.
In the season premiere Yes, Then Zero, Serena, Dan, Nate, and others receive Blair and Louis' Save The Dates that state the wedding will be held November 26, 2011 in Monte Carlo. While wedding planning, it becomes quite clear to Blair that Sophie already has decisions made while she gets little say. The last straw comes after Sophie informs Blair will have to wear her wedding dress, and Blair asks Louis to tell Sophie how important choosing her own dress is to her. He promises he will, but doesn't. Serena suggests Blair wear Sophie's dress in the photoshoot being held for Vogue Paris so she has leverage to wear her own dress on the real day. Louis eventually confesses to Blair how hard it is to say no to Sophie. Blair asks if she can come to Louis' uncle's speech at the United Nations and he agrees to let her come even though it goes against rules to prove a point to Sophie. However, he is late to pick her up due to helping Dan prevent the release of an exposing article about Blair. Blair gets upset, as she thinks Louis can't even stand up to his own mother for her. She's prepared to break it off and leave with Dan until it comes out why Louis was really late. At the end of the episode, while preparing for her Vogue Paris shoot, a stylist asks how far along in her pregnancy she is. Blair denies it but the stylist reminds her that she will be showing by the time of her wedding. At the same time, Louis brings peonies to Blair, despite the fact that brides in Monaco walk with carnations, and tells Sophie there will be other changes to be made as well.
Louis' sister Beatrice Grimaldi arrives in Beauty and the Feast to escort him back to Monaco for a feast to be held. At the same time, Blair is battling morning sickness and is happy to see them leave so she can recover. However, Beatrice has different plans and informs Blair she's staying the whole day so they can hang out and bond. After Blair has to excuse herself several times, Beatrice assumes she has an eating disorder and informs Louis. Louis doesn't believe her, but Beatrice insists she can prove it. To do so, she gets the feast moved to New York so Blair can attend. At the feast, Beatrice overhears Blair tell Dan she's pregnant but decides to help Blair by not telling Louis. In The Jewel of Denial, Dan convinces Blair to take a DNA test to find out who the father is: Chuck or Louis. Blair does so unwilling but puts off reading the results for as long as possible. While being interviewed, a reporter asks when she and Louis plan to have children. Blair is unsure of how to answer, but the reporter tells her Louis' answer was that he sees parenthood as a long way off and he doesn't think Blair is ready either. She eventually reads the results and finds out the father is Louis. After finding out for sure, she reveals her pregnancy to Louis, who is surprisingly very happy. He admits that he said what he did in the interview to save Blair from being on constant baby bump watch and controversy. Blair hides the paternity test results in her desk.
In Memoirs of an Invisible Dan, Dan's book Inside is published and released. Blair and Louis are set to have dinner with Sophie, Eleanor, and Cyrus that evening where they plan to reveal Blair's pregnancy. But Louis gets ahold of Blair's copy and reads the parts about her, where he discovers Dan wrote about having sex with Blair. Upset, he tells Blair that Sophie was delayed leaving Monaco and won't be able to make it and they'll reschedule for another time. Eventually, Blair realizes Louis was upset because he thinks she had sex with Dan. The two argue at Dan's book party and Louis leaves. The next day, he apologizes for not believing Blair and they make up.
Blair and Louis reveal her pregnancy to the family in The Fasting and the Furious. Sophie and Cyrus are delighted, but Eleanor is more skeptical. Blair thanks Beatrice for keeping her secret. While sitting down later to discuss the baby's upbringing, Sophie informs Blair that the baby is to be born and raised in Monaco, along with attending boarding school in Switzerland. Beatrice mentions to Sophie that Blair is uncooperative and is only interested in getting her own way. After feeling like no one in New York, including Serena, is on her side anymore, Blair tells Louis to tell Sophie that she's prepared to move to Monaco and live like a Grimaldi. Meanwhile, Eleanor accidentally plants the idea of a binding contract into Sophie and Beatrice's heads. At the Yom Kippur dinner, it's revealed Sophie did indeed have a contract made up. Upon reading the clauses, it's shown that some are pretty steep, including Blair not being able to leave the palace without informing security, and her not being able to go to New York for more than 48 hours at a time without losing custody of her child to the Grimaldi family. Louis confronts Sophie about it, who denies putting the strict clauses in but admits to having the contract drawn in the first place. They realize Beatrice put them in, but Sophie decides to side with her daughter, stating that if Blair really meant what she said she would have no problem signing. She says that if they don't sign the contract, she will not acknowledge Louis or their child as her heir. Louis replies that she doesn't have that right anymore, as it's the child's right to acknowledge her.Later, Louis tells Sophie he wants her to not make the mistake. She replies that he's mistaken, and he shouldn't abandon his family for Blair. Louis says he trusts Blair, but Sophie says that she hopes that that is true and she leaves. After the party, Blair apologizes for making Louis choose between her and his mother. He replies that she is his only family now, and when Blair goes to take a bath, it's revealed that he found her DNA test results. He calls Dr. Eliza Barnes, Chuck's new therapist, and says he has a problem he needs to discuss with her.
In I Am Number Nine, Blair holds a competition among her former minions to choose her bridesmaids. Meanwhile, Louis waits outside Dr. Barnes' office for her, along with a check for what he owes her. He reveals that he is paying her to turn Chuck back into the monster he was to ensure Blair will never fall for him again. She goes to meet with Chuck at his suite at the Empire, under the guise that their last appointment ended too abruptly. She brings up Blair, and asks if Chuck is angry that she is marrying somebody else. He replies that he let Blair go and that's the end of the story. She presses on, saying Blair is the source of his suffering, and that there's no way he can't not be angry. She tells Chuck that he has to accept that he didn't let Blair go, he lost her to someone else. He gives the engagement ring he bought for Blair to Dr. Barnes and asks her to get rid of it, and she takes it and leaves. In the elevator, she texts Louis that she can't do it anymore. Later, Louis surprises Dr. Barnes at the dog park. She tells him once again that she can't destroy a man who is doing everything he can to heal himself. Louis asks about their deal, and she returns the check. Unbeknownst to the two, Chuck is also at the park and overhears their entire conversation. He calls Dr. Barnes and tells her that he thinks he's in trouble and asks her to meet him later that night. At The Spectator launch party, Chuck reveals to everyone that Louis was paying Dr. Barnes to reveal confidential information about him. Blair demands to know why he's doing this and Chuck tells her to ask a very guilty looking Louis. Blair and Louis step outside of the party and she asks him why he felt the need to pay Chuck's therapist for information. He tells Blair that he found the paternity test she had done. Blair explains that she had the test done so they could both have peace at mind, but Louis thinks the test was proof she still felt she had a choice to make between him and Chuck, and he needed her to see that Chuck will always be the wrong choice. Blair reassures him that her and Chuck will never be able to work, as he destroys everything in his life and it's scaring her that he is turning into Chuck. At the end of the episode, Blair tells Louis that he should go on the trip he was planning, as a little space would be good for them.
In All the Pretty Sources, Louis is determined to prove to Blair that her friends don't have her best interests at heart. He overhears a conversation between Serena and Nate, where he finds out Serena has access to all of Gossip Girl's sources. He posts them to The Spectator and goes with Blair to their engagement party, which Serena has planned with a Breakfast at Tiffany's theme, which is Blair's favorite movie. However, the sources come out at the party and Blair finds out Louis released them to prove his point. Upset, she tells him she needs to think about the future of their relationship and asks him to leave.
In Riding in Town Cars With Boys, Blair and Louis haven't been photographed together in weeks and it's starting to catch the tabloids attention. Blair's pregnancy is causing her fingers to swell and her engagement ring no longer fits. Dorota tells her that it's normal, but Blair is concerned the paparazzi will take the ring absence to have a deeper meaning. Dorota says that maybe it does, since Louis has been back in New York for a week and Blair still hasn't seen him yet. Blair informs her that she will see Louis when she figures out what to do. Dorota reminds her that Chuck told her that to make Louis a good man again, she has to reassure him he won't lose her. Blair confesses that she isn't sure she can honestly promise Louis she will never leave him. Dorota asks what is stopping her, since the handsome prince who is also the father of her baby truly loves her and that it's really the thought of Chuck that is stopping her. Blair denies it, saying that while Chuck has changed for the better, her decision has to do with Louis. She tells Dorota that until she decides what to do or her finger swelling goes down, they are not leaving the building. After getting invaded by a photographer, Blair decides to stay in Brooklyn with Dan for awhile. At the loft, Dan asks Blair if everything is okay. She replies that everything is horrible because she needs to tell Louis he will never lose her but something is stopping her. Dan says it must be her feelings for Chuck, and Blair again denies it. However, she eventually admits that he is right, but now Chuck has become the man she always wanted him to be and she knows how to make Louis a good man again, so she's trapped and has to choose. She begs Dan to help, and Dan asks to make a pro/con list. She says no at first, but then decides to try. She says that a pro for Chuck is he's finally become a good man, and Dan says as a con that he's slept with everyone in Manhattan. For Louis, he's a prince, but Dan adds he's only the prince of Monaco. She lists how he's the father of her child, and the fact is that that will always have to come first and the only ending to her fairy tale. She calls Chuck, who tells her that marrying the father of her child can't be a mistake. Blair, devastated, hangs up. Meanwhile, Dan and Serena are at odds and Serena decides to help Louis get Blair back. Louis tells Serena how he's going crazy from not hearing from Blair. Serena offers to help, and Louis is confused as to why Serena would want to help him, but Serena insists that he is the only one who can make all of Blair's dreams come true. She makes Louis promise not to scheme anymore before telling him where Blair is. At Charlie's party, Serena walks up, looking for either Blair or Dan, and she, Charlie, and Nate see Dan lead Chuck into a back room. Serena asks what he's doing, and Nate says it looks like he's helping Blair and Chuck finally be together. Nate mentions to Charlie that he hopes the paparazzi don't find out Blair is here, or they could ruin the party by trying to get a photo. Charlie asks if they really could ruin the party, and Nate says yes before going to warn security about Max. She sends a tip to Gossip Girl about Blair being there. Inside, Chuck tells Blair that he had it wrong, that just because Louis is the father, that doesn't mean Blair needs to pick him. She asks what he means, and Chuck replies that he will love her baby as much as he loves her and that it would have been selfish to break up her family. The two leave the party, and in the car, Blair reveals to Chuck that she is ready to leave Louis to be with Chuck. Before she can end it, the car crashes, leaving Blair and Chuck badly injured.
In The End of the Affair? Blair and Louis are still together and have returned from Europe, unbeknownst to Chuck. After a chance meeting on a walk, Chuck ends up at the Waldorf's with Louis. While he goes off for a second, he runs into Blair, who leaves immediately. Louis admits to Chuck that Blair keeps going on late night excursions and he has no idea where she's going or what she's doing. He says he wants to hire a private investigator to find out; but Chuck says he has a better idea. He decides to hire Dan, who grudgingly agrees. Later at a dress fitting, Dan tries to tell Blair she shouldn't go through with the wedding but she thinks she should. He calls Chuck and tells him Blair is doing nothing secretive, but Chuck follows them to a private residence where they enter together. He takes pictures and uses them to prove to Louis that Blair and Dan are having an affair. They tell Serena, who doesn't believe them. Still, Chuck and Louis want evidence, so they ask Serena to hand over her laptop; she's been getting all the blasts that would previously have gone to the currently defunct Gossip Girl site. They find photos of Dan and Blair together and decide to confront them at the Spectator's New Year's Eve party that night. Later, Serena is over their conspiracy theories and decides to ask Blair herself. Being pretty sure, Louis adds a photo of the two together with the caption "Most Secret Affair" to the end of the year slideshow. Meanwhile, Blair tells Serena the truth. In a flashback, we see her learning that she's lost the baby and that it's not looking good for Chuck. She ends up praying in the chapel, promising God that she'll stick to her vow and marry Louis if He saves Chuck, who ends up surviving. In another flashback, it's revealed Blair is actually visiting a church with Dan and talking with a priest. After hearing this, Serena tells Blair that Chuck and Louis think she's having an affair with Dan. At the same time, Nate has found out about the extra slide. Dan tries to convince everyone it's not true until Serena arrives and says Blair is just helping them cover up their new relationship. Louis finds Blair and apologizes, and Blair says her real secret is that she's converting to Catholicism so they can be married in the church. After Blair goes to see Chuck, Louis asks if she wants to back out. Blair says she's never been more sure of anything in her life.
Beatrice returns in Father and the Bride. After getting Blair drunk at her bachelorette party, she asks why she's marrying Louis even though she doesn't love him. Blair admits that there's a reason certain loves aren't meant to be and that she does Louis even though he may not be the great love of her life. The next day, Blair confesses to Serena that while drunk she found Louis' wedding vows and read them. She explains that while she knows Louis loves her, she was never sure if her understood her, but after reading them she knows he does now. She also says that reading them is exactly what she needed to go to her wedding sure of her choice.
Blair and Louis' wedding day finally arrives in G.G. Louis is very happy and feels like the luckiest man in the world to be marrying Blair. Before the wedding, Sophie reminds him that all weddings are business moves and she never thought they would make it to the day. Louis defends their relationship and says his marriage to Blair is not a business arrangement. Meanwhile, Georgina Sparks is ready to ruin the day by any means possible. Her first plan, to seduce Louis at the church, fails so she has to find another way. She gets her chance when Chuck arrives at the church, brought by Eleanor, to talk to Blair. She videotapes Blair confessing her love to Chuck. Blair tells everyone that despite their protestations, she plans to go through with the wedding. She walks down the aisle to a smiling Louis, but before they can start vows, everyone's phones begin playing Blair's love confession. Blair flees the scene and confronts Chuck about sending the video in, although he denies it. She also tries apologizing to Louis, who looks very angry. After she pleads, he goes and talks to Sophie. They go through with the wedding and reception and things seem to be going well until it's time for their first dance. During the dance, he tells Blair that he had no choice but to go through with the wedding to save face. He also says that he's completely done with her and the only thing between them now is a contract and that their wedding is all for show. Devastated, Blair asks why he would want to be married to someone he doesn't love. He retorts that all marriages are for show and she better be ready to put on the best show everyone has ever seen until he's ready to end it. After this, Blair quietly leaves the reception with Dan.
In The Backup Dan, Blair reveals her plan to sneak to the Dominican Republic to get a divorce from Louis without his consent. Before she can leave, Louis and Sophie, after different failed efforts to find her, go to the press and try to spin it to make Blair look crazy. Eventually, Sophie catches up to Blair at the hotel she's hiding out at with Dan by the airport and tells her that unless she goes on her planned honeymoon with Louis, her family will be forced to pay the dowry that would bankrupt them. Not wanting to ruin her family's future, Blair decides to follow the rules of her prenup, which state she cannot default on the marriage during the first year, and go with Louis. When Eleanor tries to stop her, Blair explains she doesn't believe in saving herself while ruining her family at the same time.
In The Princess Dowry, Blair thinks Cyrus may have found a loophole in her prenup: that since they signed the marriage license in Monaco but married in the US, they might be able to contest it. At CeCe Rhodes's wake, Blair's royal handler informs Blair that Cyrus' plan will never work. She proposes a different plan, that she's always had a crush on Louis but her commoner status prevented them from being together, and when Blair came along and became a PR disaster, her chance to be with Louis is now. Blair admits that she just wants Louis to be happy and tells SD to make the call to the Grimaldi's. After calling, she tells Blair that Louis agreed, and she needs to get back to Monaco so she can attend a press conference where Louis will tell everyone Blair was only ever after his title. She also says the royal family's publicist will build SD up, but Sophie and the family will only grant the divorce if Blair agrees to a permanent gag order on her time with Louis. That includes posts on Page Six and Gossip Girl. Blair agrees. Outside, Georgina, who is also in attendance, overhears the conversation. Meanwhile, Chuck is confident Blair is trapped in her marriage and unable to be with anyone until Georgina tells him Blair's royal minder just brokered a deal which includes Blair getting divorced. Dorota arrives and asks Blair why she hasn't been answering her phone. Blair replies that she's at a wake and Dorota worriedly tells her that Cyrus has been trying to reach her. Blair explains that she took care of the annulment herself and Cyrus doesn't need to worry. Surprised at her statement, Dorota then asks Blair why the Grimaldi's called Cyrus demanding for all the dowry money now. Confused, Blair tells Dorota about the gag order. In response, Dorota says that maybe Sophie saw Gossip Girl and shows Blair the pic of her and Dan kissing (taken by Georgina in Crazy Cupid Love). Desperate, Blair asks SD if there's any way for the deal with the Grimaldi's to be salvaged. SD reveals that there never was a deal and that it was made up with help from Georgina. She leaves. Later, at the Waldorf penthouse, Georgina goes to see a depressed Blair. She tells Blair that she's started collecting favors because people would be coming after her when they find out she was Gossip Girl. She claims to have a way that she can get Blair her divorce and make sure the Waldorf's don't have to pay the Grimaldis a thing, so long as Blair is indebted to her. Hesitantly, Blair decides to accept the deal.
In It-Girl Interrupted, Blair's divorce papers arrive already signed by Louis. Blair lies to Dan and says that she can't sign them yet because they're missing the royal seal and the Monaco law offices have very short hours. Dan gets Dorota to let him see the papers and sees that she was lying. After talking with Nate, Dan discovers that Chuck paid Blair's dowry and that's how she got her divorce. Dan later calls Blair and tells her Chuck was the one who paid her dowry, and is surprised that Blair didn't know. She confronts Chuck, who insists he never meant for her to find out. But Blair is convinced Chuck is trying to buy her, but Chuck says if that was the case, why did he never come to collect? Dan arrives and apologizes for revealing the info, but he was worried because he also saw the unsigned papers and Blair explains it had to do with her having trouble of letting go of being a princess.
Blair: I can't live in this fairytale.
Louis: I promise, this is not a story for little girls.
Blair: But.. I've been using it to run away from my real life on the Upper East Side. Someone there did something for me that I have to honor.
Louis: Will I ever see you again?
Blair: You can always come find me.
-Double Identity (4x02)
Louis (on Blair): All my life I though I'd be married to someone I didn't love. Maybe, just maybe, that nightmare will turn into my dream.
-The Princesses and the Frog (4x20)
Blair: You either find a way to trust me, or you let me go.
Louis: I could never let you go.
-Memoirs of an Invisible Dan (5x04)
Blair (after hearing a knock at her door): Who is it?
Louis: Someone completely undeserving of your love and affection.
-All the Pretty Sources (5x08)
Louis (to Blair): After the accident, you told me you'd never leave me. Now you feel further away than ever.
-The End of the Affair? (5x11)
Louis (to Serena): I am so nervous. Can you believe it? She's marrying me. I'm the luckiest man in the world!
Blair (to Louis): Oh my god, Princess Blair, that's the first time I've heard it. Thank you for giving me another chance. Louis, the champagne hasn't gone to my head I'm too high for it to reach me. We've made it. Finally.
Blair: My whole life I wanted to be a princess. When I met Louis, it was this whole whirlwind of royalty and tiaras and it seemed like my dream was coming true. I mean, I may have married a prince but I never really got to be a princess.
-It-Girl Interrupted (5x19)