ข้อมูลอนิเมะKoi Kaze

Koshiro is recovering from breaking up with his girlfriend. On his way to work in the train, he first meets Nanoka while returning a name tag that she dropped as she was trying to exit the train. After returning to his house from work, he finds out from his father that his sister, whose face he can no longer remember, will be moving into their house for easy of communting to school. His girlfriend visits him during lunch break the next day to get back the key to her house. At the end of the work day, a customer gives Koshiro and his boss two tickets to the theme park next door. Upon leaving their work place, Koshiro meets Nanoka again and spends the rest of the day at theme park with her; they consol each other on their love lives. Their father appears at the end of the day to pick Nanoka up, revealing to both that they are siblings.

Koshiro is uncomfortable with Nanoka's doting presence as he struggles with his feelings for her. Nanoka takes over many of the households chores and even makes a bento for Koshiro. He reaches his limit when she arrives at his office to visit him, yelling at her, telling her to never visit him again and stop calling him Big Brother. To work off her anger Nanoka goes out with some friends to eat ice cream and karaoke until night. On her way back she calls once again, but is yelled at by Koshiro again. However she forgives him when he goes to pick her up on bicycle.

Unable to sleep, occupied with thoughts of Nanoka, Koshiro remembers how in his childhood what helped sleep on such sleepless nights. After he wakes up, he exchanges the bear he just brought with Nanoka's bear, both made by their mother. Her friend falls in love with a guy only to find that he is Tamaki's boyfriend. Upon returning home, Koshiro finds Nanoka taking pain killers, finding out later that she is having her period. On his day off the next day, it begans to rain, prompting him to wait for her with his umbrella.

While going to the movies with his sister, Koshiro is spotted by his Ex-girlfriend. Afraid that she got the wrong idea that he was trolling for high school girls, he meets her for dinner to explain.

Koshiro is shocked to find that Nanoka has emptied the trashcan in his room that contained the tissues of his guilt. He shouts at her for invading his privacy. That night he returns from drinking and drops and breaks her rice bowl when she surprises him contemplating it. Next day, when his father calls Koshiro at work and asks him to meet Nanoka at the station with an umbrella, he discovers that it is Nanoka's birthday.

Nanoka discovers that she has developed a reputation at school for having a brother complex. After working late on the upcoming school festival, she is walked home by a male schoolmate. Koshiro encounters them and shouts at her for being so incautious with boys. She protests her innocence and his jealousy shows when he blurts out that she has to be careful because she is cute. Next day she avoids him by leaving early and gets jostled and groped on the train. They have a talk that evening and make up. Next day, he makes a point of going with her and protects her on the train from the crowd.

Nanoka receives a love letter at school. She doesn't know what to do about it. She asks Koshiro what she should do about the love letter and he indifferently says she should make up her own mind and not consult him. She meets with her admirer and turns down his advances and then is disappointed when he doesn't seem to care. Koshiro regrets his prior indifference and tells Nanoka that if she wants advice about anything, she can ask him. She is impressed by his concern about her and says "I love you".

Koshiro remembers the birth of his sister and their life together before their parent's divorce. Although they were too far apart in age to understand each other, they felt a vague sense of affection. In the present, Koshiro goes to see his mother for the first time in many years. She tells him how much Nanoka looked up to her big brother and tells him to be a good big brother to her. On the ride back, he remembers an incident when baby Nanoka gave him a glass of mud and he ended up keeping it because it made her so happy. He resolves to be a good big brother to her.

Koshiro contemplates moving out to resolve his situation while Nanoka plans to make him a sweater for Christmas.