Meiko Shiraki is a central figure in the narrative, serving as the vice-president of the Underground Student Council. Her role is primarily defined by her position as the strict and physically imposing guard of the male prisoners, a duty she undertakes with an almost terrifying level of dedication. She is the most loyal subordinate and closest friend to the council's president, Mari Kurihara, and her actions are almost entirely motivated by a deep-seated need to serve and protect her.

Physically, Meiko is a striking presence. She is exceptionally tall and boasts a powerful, muscular physique with a notably voluptuous figure. Her long, straight gray hair is typically tied up in a bun, a style she adopted on Mari's advice during middle school. Her standard attire is the school uniform, which she wears in a deliberately provocative manner with an open collar and a miniskirt, often complemented by knee-high boots and a horsewhip she carries as a tool for punishment.

Her personality is a study in contrasts. On the surface, she is a confident, ruthless sadist and a self-proclaimed misandrist who derives a sense of purpose from dominating and punishing the five male students in her charge. She is relentless in her duties, conducting rigorous roll calls and administering harsh physical punishment, most famously with her horsewhip. However, this exterior masks a fragile interior. She was once a shy and meek girl who was cruelly bullied in elementary school by Kate Takenomiya because of her early-developing and large chest. It was Mari who saved her, encouraging her to stand up for herself and tying her hair back, a gesture that forged an unbreakable bond of absolute loyalty and near-obsessive devotion from Meiko.

This traumatic past makes her vulnerable. When she is manipulated into believing she is responsible for burning a tree containing Mari's beloved crow hatchlings, the psychological shock causes her to mentally regress to her timid childhood self, losing all of her dominant traits. Her primary motivation throughout the story is to serve Mari Kurihara, whose approval and friendship she values above all else. She is a key enforcer of Mari's schemes, most notably the DTO plan designed to trick the boys into violating their parole to have them expelled.

In the story, Meiko acts as the primary antagonist and warden for the main male characters during their incarceration. Her brutal methods, however, sometimes backfire, as the boys' perverse reactions to her punishments force the student council to change tactics. Despite her cruelty, she is not without a twisted sense of honor, granting the prisoners their requested monthly gifts and even fulfilling their last meal requests before a planned expulsion. Her key relationships are defined by her devotion to Mari, her rivalry with her former bully Kate, and her dynamic with the prisoners, particularly Gakuto and the masochist Andre, whose fetishes she cannot comprehend.

Meiko possesses immense physical strength, which she maintains through rigorous daily training involving hundreds of squats and push-ups. She is a proficient combatant and judo practitioner, with her signature leglock being a particularly devastating move. Her body also has a notably high metabolism, causing her to sweat profusely, especially during moments of extreme stress or fear. Over the course of the story, Meiko undergoes a significant psychological arc, from a confident enforcer to a broken child and finally to a restored state, ultimately reaffirming her loyalty to Mari and, later, to Chiyo Kurihara as the new student council president.