Tetsu is one of Kumiko's lackeys. He is a member of the Yakuza group, and is very devoted and protective of her. He and Minoru always keep an eye on Kumiko and do their best to help her when they can.
One day he and Minoru showed up at her house, begging to join the Kuroda clan. Due to their ages, Wakamatsu had to refuse. Although Kumiko, tried to persuade them to go home to their families. Minoru revealed both had none. Tetsu had lived with a single mother. But one day he woke to find himself alone and realized his mother had abandoned him for a lover.
Kumiko took him in when he was a 15, calling him a 'younger brother' since the family didn't accept underage kids, and they had nowhere else to go. When the two weren't playing with Kumiko, they did run errands for Wakamatsu.Though they tried finishing school, both were sent back. As their teachers found them unfit for school. Its their treatment by the faculty, that motivates Kumiko to become a teacher.