Kotomi is another schoolmate of Tomoya's in the same year, but, like Kyou, is in a separate class from Tomoya. The "Koto" in Kotomi is derived from the Japanese musical stringed instrument of the same name. She is in the top ten for every subject throughout the whole country in standardized exam results—she always goes to library to read extra materials, especially books in foreign languages. Kotomi is a very quiet girl and it is quite difficult to communicate with her. In her spare time, she plays the violin, although she is very poor at it to the point of causing pain to those who hear her play.
Tomoya meets her for the first time when he skips class. He enters the library and finds Kotomi sitting on the floor barefoot (she is often seen with her shoes removed), reading books with scissors in her hand. Tomoya stops her from cutting the book with her scissors, then strangely, Kotomi offers her bento to him, and asks him to come again the next day. When they meet again, Tomoya asks her whether she likes to skip class, which Kotomi agrees. One day, Youhei meets up with Tomoya, and is told that there is a girl who offers free food. Youhei enters the library but he gets ignored. Youhei is surprised that Tomoya knows Kotomi as she is an academic student who got top ten in the nation-wide, and is exempted from lessons. Tomoya starts skipping the fourth period and spend his time with Kotomi in the library. Tomoya later decides to have Kotomi make friends to make her more sociable.
Just when Kotomi becomes more sociable, a bus accident causes her to breakdown in front of everyone and has to return home later. Tomoya visits her in her dark house and learns that he and Kotomi were childhood friends, but lost contact with each other after Kotomi's parents died in an air-accident as their plane malfunctioned. She does not want to lose anyone she loves anymore, and asks Tomoya to leave her alone. Wanting to bring Kotomi away from her past, Tomoya decides to renovate her garden. The drama club members want to gift Kotomi a violin, but it got destroyed by a speeding biker. After renovating Kotomi's garden, Kotomi appears to Tomoya, and after some poignant conversation, Tomoya brings Kotomi out of her house to see the outside world inside her heart and returns to her usual self. The next day, her guardian presents Kotomi her parent's briefcase that has traveled all the way to Japan, and inside is a teddy bear and a letter, asking Kotomi to live the way she wants and grow up to be an adult. Kotomi realizes she is still being loved by her parents and cries. After this, Kotomi becomes more mature, and the drama club members finally give Kotomi a violin.
In the movie, Kotomi only made a cameo appearance, as she is only seen conducting a song for the members of the Choir Club. She, however, did not play any role in the movie.