First Manga Appearance: Chapter 01
Name Meaning: Mariko=marina; true reason child
Occupation: Teaching staff at Tsujido High School
Gangs: Red Rouge Ladies
Special Skills: Knows the red light district trips of the famed soapland Turkish bath girls; has a limber body that can be twisted into many shapes; knows how to apply makeup really good; demanding more
Notable Features: Lost her virginity at 13 to home tutor; goes after what she wants
Izumo Mariko is the daughter of a Yakuza boss, contributing to her wild past including her involvement with the all girl gang "Red Rouge Ladies". Due to her determination to get whatever she wants, she would shamelessly chase after any guy that she fancies (it is noted that she lost her virginity to her home tutor at the young age of 13). She eventually becomes a teacher at Tsujido High.
She goes on a holiday with her friend & colleague from Tsujido, Murakoshi Ayumi during the summer holidays to the beach resort at Yoron Island. They are harassed by guys from Chiba & are saved by 2 waiters, Onizuka Eikichi & Danma Ryuji, who claim to be medical students, while Mariko & Ayumi pretend to be 2nd year college students. Mariko takes an interest in Eikichi while Ayumi is paired up with Ryuji. She almost sleeps with Eikichi but is interrupted by his bad stomach. Due to a fight at the disco, she discovers that Eikichi & Ryuji are just high school delinquents.
On the 1st day of school, she is shocked to find that Eikichi & Ryuji are new transfer students at Tsujido & begins to plot her revenge with Ayumi against them. When the Oni-baku blackmail her & Ayumi to have sex with them or else they reveal the Yoron Island incident, she lures them in & calls her father's homosexual Yakuza members to take her place. She eventually saves Eikichi from the homosexual Yakuza, tends his wounds & praises his determination.
Sometime later, she witnesses Eikichi's involvement with Aina. She goes on the Christmas ski trip & offers to teach Eikichi how to ski upon seeing his poor skills. She later saves Eikichi from freezing after Aina leaves him & tells him to go save her as she really loved him. As Eikichi rushes off, she smiles & realises why Aina would like Eikichi so much.
When Saejima Toshiyuki & Kamata Jun return to Tsujido, she was the first to warn the Oni-baku of their return & advises them not to mess with the Kamakura Mad Dogs. She worries about her students during the Oni-baku & Kamakura Mad Dog fight & tries to stop Eikichi from confronting Saejima.
During the Shindoji Fumiya arc, she acts as Ayumi's support & advises her to confront Fumiya & tell him the truth. She witnesses her students getting beaten up by Kyokuto High & rushes to their aid. She witnesses Ayumi getting kidnapped by Fumiya & informs Ryuji to save Ayumi.
When Ayumi & Ryuji decide to get married, she congratulates them & asks Ryuji to take care of Ayumi. Ayumi later confides in Mariko about her decision to leave Ryuji to allow him to enjoy his youth & she is present when Ryuji rushes to the station in attempt to stop Ayumi from leaving. She later comforts Ryuji & reminds him that it was hard for Ayumi as well & that they are both lucky to have experienced something that they can cherish forever.
When Minamino Youka arrives at Tsujido as a teacher, she reveals his weakness to Eikichi, Saejima & Abe, helping them stand up to the brutish teacher. She later informs the Oni-baku of Atsushi’s attack by Yagyo Atsuki. She cooperates with the Juvenile Department to question students on the string of Yagyo Atsuki attacks.