Kyo is a tough-looking member of the Yakuza, but he is very close with Kumiko. When he was younger, and Kumiko was a little girl, he taught her how to fight and defend herself when she was being bullied in school and teased because of her Yakuza connections. (Source: Wikipedia)

Kumiko "Yankumi" Yamaguchi enters Shirokin High School to become a math teacher. She's assigned as the homeroom teacher for class 2-4, populated by a gang of delinquents.  At first, the class tries everything to rattle her cage, but to no avail. In a short period of time, Yankumi gains the respect of the class. But what the school doesn't know is that she's the heir to the Oedo Group, a powerful yakuzan clan. So while Yankumi struggles to excel in her career as a teacher, she must also keep her yakuza background a secret.<br>
<br>
(Source: Media Blasters)

[Wiki (jp)](https://ja.wikipedia.org/wiki/若本規夫)

Used to work for the Tokyo Metropolitan Police Department Riot Police.

Fresh out of college, Kumiko "Yankumi" Yamaguchi becomes a math teacher at an all-boys high school. Unfortunately for her, she's assigned by the school board as the homeroom teacher for class 3-D, which happens to be the class of delinquent students. When the class attempts to make a living hell out of Yankumi's career, they fail as her persistence and aggression gain her respect. However, little does anyone in the school know that Yankumi is actually heir to the Kuroda Ikka—one of Tokyo's most powerful yakuza clans.
<br><br>
(Source: Anime News Network)

Note: Chapter count includes 9 extra chapters in Volumes 9-15

Kyotarou Ooshima