He's a 17 year old boy that goes to the Kibina High School with his friends Kazuki and Hiiragi. Mao is a childhood friend of Kōichi who went overseas to Paris to live with her parents but came back two years later. Mao came to kibina to live with her childhood friend and his mother and father and to find that Kōichi forgot all about her. It took him a while to remember her when Kazuki came to get him for a pr movie to welcome newcomers to the Film Research club. He recognized her instantly and moments later he realized who she was. His initial love interest in the anime is Yūmi Hoshino, but he later falls in love with Mao. In the manga version he likes all of the girls (Mao, Yuumi, Eriko, Asuka and Mitsuki.)
He is friends with Kazuki Aihara, Akira Hiiragi and Mao Mizusawa, who recently came back from two years in france. He started to date Yūmi during summer vacation because of Mao. He calls Mao "Big sister Mao" (摩央姉ちゃん Mao-neechan) because she was like a big sister to both him and Kazuki.
When Yūmi told him that she was transfering schools, he promised her that he will be with her even if she tranferred, but at the school festival he broke that promise. He told her that he was in love with Mao and, when he told her that, she said she wanted him to tell her that from the beginning when he first devoloped feelings for her and that she knew that he had been devoloping feelings for Mao. After that she left him in tears so he ran all the way to Mao to tell her that he loves her. She said that she loves him too and they kissed. Then the fireworks went off at the school then they watched them and Mao turned to him and they kissed again.
In the game, he and Kazuki were one person but in the anime they were split into two characters. Kazuki got Kōichi Aihara's soccer skills and family name and Kōichi got his good looks, given name and design.