Age: 6 (Digimon movie part one), 10 (Digimon 01), 13 (Digimon 02), 16 (Digimon Tri.)
Birthday: November 6, 1989

Koushirou is a computer expert and spends most of his free time researching on his laptop (which resembles a Tangerine Apple iBook, only with a pineapple symbol). He is very curious and is always making theories and trying to understand the world around him. He starts out as a very introverted person, who isn't very in touch with other people and doesn't really understand their feelings. Being logical and thoughtful, he doesn't appreciate impulsiveness and gets angry when people around him act before thinking. He used to be on the soccer team with Sora and Taichi, though this was only to avoid contact with people as he isn't very good at playing soccer. He is the partner of Tentomon and the bearer of the Crest of Knowledge.

Koushirou's biological father was a genius mathematician and a lecturer at a university; he and Koushirou's biological mother died in a car crash when he was a baby. He was adopted by Izumi Masami, a distant relative of his father's and his wife Yoshie, who had suffered the loss of their own baby boy not long before the accident. Koushirou was unaware for most of his life, and once he found out, he was devastated. However, he had since learned to accept himself as a true Izumi.