According to the anime version, Yuu meets Miki before even knowing who she is, and she captures his interest as she works hard on her tennis and chases after the little kids as the "towel monster." She really sparks his interest when he finally realizes who she is. "So this is the girl I'm about to live with.."
He realizes his feelings for Miki long ago, but he does not reveal his emotions due to fear of being betrayed. After his parents' incidents and his belief that he is not Youji's biological son, Yuu cannot trust anybody readily anymore. He does tease Miki a lot, but always comes through when she truly needs him. When Miki became upset at the trick her parents played on her and barricaded the door to her room, it was Yuu who climbed from his balcony to hers and comforted her. Yuu helped her calm down before her big tennis match (anime) by taking her shopping. Arimi tells Miki that she notices a soft expression in Yuu's eyes whenever he is around Miki; an expression he never had before. He has never been the type of guy to open up to anybody, but he seems to open up and show his true side to Miki. This observation is incorrect..as Miki sees after she becomes conscious of the fact that Yuu never really shares anything about himself to her. She's lived with the guy for over a year, and she doesn't know anything about his dreams or likes or anything. There's a wall between the two that must be overcome before they can progress in their relationship. Yuu originally does not plan on telling the secret of his birth to anybody, but after realizing that it was more painful keeping it a secret from Miki and risking their relationship, he confesses everything. What Miki and him share meant too much for him to lose.
Miki first meets Yuu at the restaurant where the Koishikawas have dinner with the Matsuuras. She immediately notices his handsome features, even during her anger at her parents for the entire divorce/switch partners plan. When Yuu kisses her in the infirmary as she pretends to be asleep (That famous scene), his actions really confuse her. Yuu acts like his normal self the next day after the kiss, causing further confusion for Miki. At an amusement park a few days (weeks?) later, Miki is shocked by the news that Yuu used to have a girlfriend - Arimi. Yuu tells her though that they did nothing and that his first kiss was with Miki in the infirmary. "Why?" "Because I like you." Miki takes his response as another one of his jokes, and slaps him in anger. Despite that, her feelings for Yuu slowly grow..she realizes that beneath his joking countenance, he is really a kind and thoughtful guy. But Miki still has another guy problem to worry about...Ginta. Her best guy friend whom she had a crush on in Junior High. She must decide which guy she likes more; which guy makes her heart pound with fervor.
Miki realizes that her feelings for Ginta are different than the ones for Yuu during Meiko's incident with Namura. As Yuu takes her to the train station to stop Meiko from a possible elopement with Namura, Miki realizes that the only one for her is Yuu. Meiko told her that "Yuu was like a breath of fresh air for Miki", but she always thought that Miki would eventually go back to Ginta, the guy who had always been there for her. However, Yuu is more than a temporary breath of fresh air. Miki needs and loves him. He's the only guy that makes her feel the way she does.
And there we lead up to the beach scene..where the two officially become a couple. Yuu finally opens up his heart, Miki confesses, and fate has accomplished its work once again.