Mariko Shinobu is a 16-year-old first-year student at Seiran Academy. Her troubled family background profoundly shapes her personality and actions. Her father, Hikawa Shinobu, an erotica author, engaged in an affair with an actress, leading to divorce from Mariko's mother, Hisako Shinobu. This instability forges Mariko's deep distrust of men and intense fear of abandonment, manifesting as obsessive tendencies toward those close to her.

Physically, Mariko possesses a striking appearance: long black wavy hair, straight bangs, grey eyes with long eyelashes, and bright red lips from habitual lip-biting. She frequently wears purple knee-length dresses and applies red nail polish to her thumbs and pinky finger. A vanity about her looks contrasts with her disregard for personal health during emotional distress.

Mariko exhibits extreme emotional volatility, marked by possessiveness and jealousy. She fixates on Nanako Misonoo as her desired best friend, manipulating situations to isolate Nanako from others, including spreading lies to sabotage Nanako's friendship with Tomoko Arikura. This culminates in a traumatic birthday incident where she confines Nanako against her will in a hotel room, threatening violence if Nanako leaves. After Nanako escapes, Mariko engages in a self-destructive hunger strike, collapsing from anemia and requiring hospitalization before reconciling with Nanako.

Her admiration for Kaoru Orihara is significant; she cites Kaoru's strength and emotional support as reasons for her deep affection, openly admitting to loving her. Mariko's intense reactions extend to bullying; she physically attacks classmate Aya Misaki after public mocking of her family issues, resulting in school suspension and expulsion from the elite Sorority group. This incident indirectly fuels later efforts to disband the Sorority.

Following her parents' divorce, Mariko experiences heightened self-loathing and shame over her father's profession. Her emotional recovery begins when Takashi Ichinomiya (brother of Sorority president Fukiko) gives her a sincere novel by her father about familial love and living without regret. This allows her to reconcile with her father's legacy and gradually accept her family's fractured dynamics.

Across media, her development varies. The manga leaves her manipulation of Tomoko largely unaddressed, and she later adopts her mother's maiden name, Sekiya. The anime portrays greater accountability for her actions, including a stronger reconciliation with Nanako post-hospitalization and eventual friendship with Tomoko. The manga hints at a potential romantic future with Takashi, while the anime leaves this unresolved. Her journey concludes with tentative emotional stability, moving past her worst self-destructive impulses while retaining passionate loyalty to close friends like Nanako.