Marina Akitsuki is Airi Akitsuki's older sister, attending the same school as a third-year student. Her half-French, half-Japanese heritage shapes her distinct appearance. Proficient in household management, she oversees chores and supports her stepfather Kouzou Akitsuki during her mother Kayoko's absences, displaying sincere and helpful traits while actively mediating tensions between Airi and Kouzou to maintain family harmony.

As Kayoko's biological daughter and Kouzou's stepdaughter, Marina becomes a target when Kouzou, motivated by sexual interest in both sisters, deploys an experimental aphrodisiac spray for non-consensual encounters. This escalates after Kouzou discovers her boyfriend Masato's predatory behavior, prompting protective panic that culminates in him pharmacologically coercing her virginity.

Marina's trajectory evolves from victimization to developing complex, seemingly genuine affection for Kouzou. She competes with Airi for his sexual attention, participating in public encounters, park intimacies, and multi-partner scenarios involving Airi and others like Sana. She joins Kouzou and Airi on a hot spring trip, though some accounts note last-minute plan alterations by her.

Despite rivalry with Airi, their closeness persists, occasionally leading to collaborative sexual situations with Kouzou. Marina becomes pregnant from her ongoing involvement with him, ultimately positioning her as a maternal figure. Kouzou exhibits protectiveness by facilitating Masato's incarceration following harassment.

Visually, she has brown hair, blue eyes, and wears standard school attire often accessorized with bows and knee-high stockings. Her narrative consistently engages themes of familial obligation, manipulated consent, and evolving relational dynamics within the household.