A Kokuhaku Jikkou Iinkai character. He is a Sakuragaoka High School student in Class 1-1, and a member of the "Idiot Trio" consisting of himself, Enomoto Koutarou, and Shibasaki Ken. His voice actor is Matsuoka Yoshitsugu. Lens Koshi no Keshiki is one of the HoneyWorks song focused on him.
Koudai has black eyes and short black hair that resembles bowl cut with arched bangs. He wears black-framed glasses. Like the others, his school uniform consists of a blue blazer, a white collared shirt, a red necktie and blue pants.