Mr. Koizumi is the Katase High School Table Tennis club advisor. He recognizes Smile's gift and ability towards the sport and so vows to train him. When Mr. Koizumi was younger he was considered to be the best table tennis player in Japan, and was nicknamed 'Butterfly Joe' because of his extraordinary skills. Mr. Koizumi states that Smile reminds him of his younger self. He is also an English teacher.