Kotoko is Hikari Hinomoto's best friend, having known her since middle school. Kotoko has a serious personality, and she is a lover of Japanese culture who also hates anything foreign. She hates Katakana, and this seems to be influenced by her brother who loves heavy metal.
She is usually cool-tempered, but can have explosive fits of anger towards people who annoy her. She is very protective of Hikari, and will confront the protagonist if she feels he's not treating her well. If the protagonist gets closer to her, she'll reveal her attitude that makes her Hikaris best friend. This side of hers is particularly emphasized in a substory of Memories Ringing On.
- Her hobbies are reading while drinking tea, writing haiku (a form of traditional Japanese three-line poetry) and senryuu (another form of Japanese poetry similar to haiku).
- She likes Japanese tea and sweets, wabi-sabi and fuuryuu (both are Japanese aesthetics.) She dislikes spicy food, coffee, western culture, pointless European writing.
- When the protagonist calls her to invite her out on a date when her evaluation level is low, she'll tell him: "Oh, so you were alive."
- The concept of a cheerful main heroine and an angry best friend have inspired Maki Hoshikawa and Tsugumi Godou from Tokimeki Memorial 4.