Teppei is the 4-year-old little brother of Fumino. Teppei's personality is bright; he often makes people around him happy. Both Fumino and Kazuma love him. Teppei views Kazuma as a brother or a father figure. He often gives Teppei candy for his hard work or just simply for being cute.
Though still very young, he often tries to defend his sister when anyone bullies her. This is shown when Shōma was criticizing her; Teppei defends her by punching him in the leg and saying, "Shō-chan, you bullied Bun-chan" with his fists balled up in anger.
When Fumino and Teppei leave Kazuma, Teppei leaves a trail of his collected hard candy for Kazuma to follow. He was also the one who tried to convince Fumino to stay with Kazuma when she tried to leave, which causes enough distraction for Kazuma to catch up with them. He loves living with Kazuma.
He gave his first drawing to Kazuma as a birthday present. It was a picture of Kazuma himself.
Teppei goes to school with his kindergarten teacher Ryū, who is Kazuma's best friend and neighbor. Teppei nicknames Fumino "Bun-chan" and Kazuma "Mā-kun."