Kumiko Yamaguchi is a math teacher with a secret: she is the 4th generation heir to a Yakuza clan.
At the age of seven, Kumiko Yamaguchi's parents died and she was sent to live with her grandfather Ryuichiro Kuroda, the third generation Oedo Yakuza boss. She developed a close bond with all the members of the group and considers them her family. She decided to become a teacher at the age of sixteen, wanting to be a teacher for the kids that most gave up on.
Kumiko Yamaguchi was created by mangaka Kozueko Morimoto. In the anime, she is voiced by Risa Hayamizu in the original Japanese and Carrie Keranen in the English dub. She is portrayed by Yukie Nakama in the live action dramas.