A university graduate, and a newly hired teacher at Shirokin Gakuen. She is very idealistic, and wants to inspire her students and help them graduate high school. Since the age of 7, when her parents died, Kumiko has been living with her grandfather, who is the head of a yakuza group. Despite this strange upbringing and the bizarre mannerisms she has picked up from it, like her use of yakuza slang and a habit of running from the police, Kumiko is a kind girl who has a strong faith in her students.

(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. 
 
(Source: Media Blasters)

Carrie Keranen is an American voice actress, production manager, producer and voice director. She is best known for her work with 4Kids Entertainment, Bang Zoom! Entertainment, DuArt Film and Video, NYAV Post, Studiopolis and Funimation. Her roles include Satsuki Kiryuin in Kill La Kill and Casca in Berserk. In cartoons, she voices Alya Césaire in Miraculous: Tales of Ladybug & Cat Noir. In her live-action work, she has played Piper Tate in Untitled Web Series About a Space Traveler Who Can Also Travel Through Time.

(Source: Wikipedia)

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.
 
(Source: Anime News Network)

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

Gokusen Kanketsuhen is a collection special Gokusen chapters. 
1. Gokusen (ごくせん2008); You 2008 #2 
Kumiko has trouble with her new class of first years. 
2-4. Gokusen Kanketsu-hen (ごくせん完結編); You 2009 #4-16 
Kumiko and Shin's relationship hasn't gone very far. Shin gets kidnapped by a daughter of another yakuza group with intrusions of making him her bride-groom. Shin's father informs Kumiko that although he respects her as a teacher, he does not want her to see Shin again. Then it only gets worse when the other former Shirokin try to rescue him before Kumiko gets there. 
5. Gokusen Bangai-hen: Ucchi & Shiori Kyoufu no Ippaku Ryokou!! (ごくせん番外編 うっちー＆詩織 恐怖の一泊旅行!!); You 2007 #15 
Chapter focusing on Ucchi and his girlfriend. 
6. Gokusen Bangai-hen: Hontou ni Attara Kowai Hanashi Gekijou (ごくせん番外編 本当にあったら怖い話劇場); Bessatsu You 2007-11-20 
Assortment of short stories with a horror theme.

The Gokusen Extra Chapter is a side story included within the Gokusen Official Guidebook. It shows the first meeting between Kumiko and Shin years before the main story.

Kumiko Yamaguchi