The Underground Student Council became a prisoner.

Mari Kurihara is a third-year student in attendance at Hachimitsu Academy and the president of the Underground Student Council. She is also the daughter of the school chairman and the older sister of Chiyo Kurihara. Mari is a popular student due to her wealth, beauty and social power.

Mari is a tall and lithe young woman who is frequently described as one of the most beautiful students in the school with back-length black hair that has two front shoulder-length hair strands hanging in the front with bangs swept to the right side, blue eyes (green eyes in the anime) and full pink lips.

She wears the standard female school uniform with dark tights.

During her incarceration, she wears an undersized female prison uniform consisting of striped short tank top and matching short striped trousers.

Mari is a proud and dignified student who exerts a sense of authority about her that lends to her popularity. Mari prefers the company of crows to people outside of her subordinates and closest friends that are senior members in the Underground Student Council, and she can control them.

Mari is well known for her intelligence, athletic ability, leadership abilities and all-round talent both in her middle and high school years. She is a calm and controlled character who displays intelligence and cunning throughout the series, as shown by her plan to exclude the boys through DTO and her later secret engineering to prevent the official Aboveground Student Council from destroying the USC with their subterfuge. Though Mari's strained relationship with her perverted father results in her misandry treatment of the boys, she adapts to be able to socialize with the boys and empathize with them.

However, Mari is a genuinely kind person deep down, as shown when she helped a younger Meiko Shiraki to become more confident and prevented her Kate Takenomiya (Mari's to-be arch-rival) for bullying her in middle school. Mari is also very close to Chiyo Kurihara (Mari's younger sister), whom she shared her childhood with even as she grew estranged from their father. She is also a graceful loser, as shown when takes punishment from the official Student Council and responsibility for physically abusing the 5 boys during their prison term, even praising Kate for her strengths though she is her arch-rival.

Mari is first introduced as the 20th president of the Underground Student Council whose vigilante organisation, the USC, polices the traditional school rules of the private school above its own policies of not speaking or entering into illicit relations with the boys. Thus, she clashes with her father, the Chairman, who supports male enrollment at the school and engages in an ass-fetish. When the boys intrude into a female-only bathing area, she rounds them up and incarcerates them in the Prison Block. There she co-ordinates the guard system, the security and their punishment (hard labor for one month). Mari continually increases the harshness of her punishments when she accidentally stumbles onto her father's pornography stash and online videos in his office.

When Mari discovered that Kiyoshi Fujino escaped prison just so that he can have a date with her sister Chiyo Kurihara, she tried to make Kiyoshi sign the school withdrawal form in lieu of one month extension of his prison sentence, but failed. Hence Mari began Operation Boys Expulsion, code named DTO, a conspiracy where they manipulate the boys to commit two more breakouts in order to gain the school's support to expel them. Mari entrusted Meiko Shiraki with the groundwork, which initially backfired with Takehito Morokuzu (Gakuto) but eventually succeeded with Reiji Andou (Andre) and Shingo Wakamoto. With DTO declared a success, the USC had an early celebration and they destroyed all hard copies of the conspiracy. The celebration proved to be premature when Mari found out that the Chairman gave the boys one-day extension to their imminent expulsion. Angered, Mari delegated Hana and Meiko with the task of stepping up the security on that final night to prevent the boys from doing anything that can help overturn the expulsions, and also participated in the monitoring herself. Despite the USC's last ditch efforts, Gakuto obtained the email evidence of DTO, by enlisting outside help from Chiyo and Anzu Yokoyama. The boys successfully exposed the conspiracy in front of the Chairman, forcing Mari to admit her role in it. Mari promptly accepts responsibility for the USC's actions and prevents Hana Midorikawa from beating Kiyoshi up.

The Chairman placed Mari and the USC on house arrest, subsequently agreeing with the Aboveground Student Council's proposal for a one-week imprisonment of the USC for their mistreatment of the prisoners. The ASC President Kate Takenomiya, who held a grudge against Mari ever since middle school, plotted to take everything away from Mari as part of her revenge and power-grabbing attempts. Kate even stripped Mari's bra during the imprisonment to humiliate her. Anticipating this, Mari secretly installed Hana as a mole in the ASC. True enough, Kate began a smear campaign against the USC, framed Meiko for starting a massive fire, extended both Mari and Meiko's prison sentence by another week, and announced her intent to permanently disband the USC. Kate also sent Kiyoshi to prison, where she manipulated Mari and Kiyoshi into compromising positions, giving her blackmail material. Through this leverage, Kate used the blackmail material to force Mari into a final cavalry battle, facing the ASC.