Kotomi is one of Tomoya's schoolmates, but in a different class. She is extremely intelligent, and is in the top ten for every subject throughout the whole country. Kotomi enjoys playing the violin, but is so bad that she inflicts pain to anyone who listens to it.
In the Clannad anime
Kotomi first encounters Tomoya while he is skipping class. He goes to the library, where he finds her sitting on the floor, barefooted, reading books. She has a scissor in her hand, and Tomoya prevents her from cutting a book. Kotomi offers her lunch to him for no apparent reason, and asks him to return the next day, which he does. Later, when Sunohara meets Tomoya, he is told there’s a girl who offers free food. Youhei goes to the library, but is ignored. He is very surprised that the delinquent Tomoya knows Kotomi, since she is very intelligent and is in the top-ten nationwide. The reason she doesn’t go to class isn’t that she skips it; she is exempted, since she is so far ahead of the other students.
Tomoya returns every day, spending time with Kotomi. After a while, he decides she needs friends, to make her more social. After he manages to get her some friends, a bus accident causes her to breakdown in front of everybody, and returns home. Tomoya visits her, and learns that they were childhood friends, but lost contact after Kotomi’s parents died in an airplane-accident. She asks him to leave her alone as she doesn’t want to lose anymore of her loved ones, and locks herself up, refusing to leave the house. Tomoya decides to fix her mismanaged garden to cheer her up. The drama club members decides to give her a violin, but it gets destroyed by a speeding biker. When her garden was fixed, Kotomi appears to Tomoya, who brings her out of the house.
The next day, Kotomi’s guardian, who she had been scared of, calling him the “bad guy”, presents her with her parent’s briefcase, which had been recovered from the crashed plane. Inside lies a teddy bear and a letter, asking Kotomi to live the way she wants and become an adult. Kotomi, who had wished for a teddy bear for her birthday long ago, realizes that her parents still love her and breaks into tears. After these events, Kotomi becomes more mature, and is given a violin by the drama club.
In the Clannad movie, Kotomi only makes a cameo appearance, conducting a song for the Choir Club.
Clannad is an anime movie based on the game by Key. This movie is not relevant to the anime but creates an alternative storyline.