Louis and Blair met at the Musée d'Orsay in Paris at the beginning of season four, where Blair is staring at her favourite painting by Manet. Louis approaches her (being a Manet lover himself) and asks her out on a date.
Their short-lived romance ends when Blair returns to New York, but she leaves Louis one of her shoes, stating "you can always come find me."
Towards the end of season four, Louis arrives in Manhattan with Blair's shoe and they start dating again. They get engaged and leave for Monaco for the summer together. He and Blair were expecting a child.
In season five, Blair loses the baby in a car accident, but she and Louis stay together. On their wedding day, a video of Blair confessing her love to Chuck is uploaded to Gossip Girl in the middle of the ceremony. However, the wedding proceeds as planned, but during their last dance before departing for their honeymoon, Louis tells Blair that he is done with her. In his ultimatum, Louis instructs Blair to smile and act happy in public, but in private, they will be like strangers.
Louis: "I promise this is not a story for little girls."
4x20 − 'Je t'adore'
4x20 - Blair to Serena: "It's official, we should send out change of address cards, because I'm moving to cloud nine. Louis is incredible, with him by my side I can actually be a powerful woman."
4x20- Blair to Serena: "Louis made me happy. Happy. Do you know the last time I felt joy? Chuck had brought me into his darkness for so long, I'd forgotten what that felt like."
4x20 - Louis to Blair: "I want you to choose joy in your life. More than that, I want to be the one who gives you joy."
4x21 - Louis to Blair: "I want you Blair. But the only way it will work is if you show me all of the parts of yourself. Even the ones you're ashamed off"
4x22 - Blair: "Louis and I it's different. It's lighter, we're simple, makes me happy."
4x22 - Chuck arrives at an empty Constance to find Louis is still there.
Chuck: "You're still here"
Louis: "Why wouldn't I be?"
Chuck: "Party's almost over."
Louis: "I know but Blair hasn't arrived yet"
Chuck: "How can you be sure she's even coming?"
Louis: "Because I love her. And just as important, I believe in her. I know she's scared, but she'll figure it out. And I'll be here when she does."