Maria Mizuse is a member of the management team overseeing the Tomodachi Game. She serves as an observer assigned to Yuuichi Katagiri and temporarily joins the C team as part of her mission to study him up close. Her background prior to this role is not detailed, but her position indicates a deep involvement with the game's administration.

In terms of appearance, Maria is an attractive young woman with peach-colored hair and emerald green eyes, often seen with her hair tied in twin tails using black ribbons. She typically wears a school uniform with boots and a garter belt, giving her a youthful and approachable look that helps her blend in as a participant.

Personality-wise, Maria presents a cheerful, innocent, and shy facade that initially makes her seem harmless and friendly. However, beneath this surface lies a twisted and manipulative nature. She is highly observant and fascinated by Yuuichi's cunning and unconventional tactics, referring to them as "monstrous." Her true motivations stem from a professional curiosity and enjoyment of the psychological games, rather than any personal vendetta or loyalty to the participants.

Her role in the story is primarily that of an infiltrator. During the Hide-and-Seek game, she inserts herself into Yuuichi's group to gather intelligence for the management. She maintains her cover by acting bubbly and engaging in casual conversation, using these interactions to assess the participants' emotional vulnerabilities and group dynamics. She reports her findings back to the higher-ups, ensuring the game proceeds as intended.

Key relationships include her interactions with Shiho Sawaragi, whom she befriends to gauge loyalty, and Makoto Shibe, whom she engages with playful energy to analyze his straightforwardness. Her relationship with Yuuichi is the most significant: she is drawn to his unpredictability and skill, and she subtly tests his strategies through banter. Ultimately, she remains loyal to the Management, which defines her duplicitous actions throughout the series.

In terms of development, Maria's fascination with Yuuichi deepens as she witnesses his schemes, though her fundamental allegiance to the game's administration does not waver. Her notable abilities include deception, psychological observation, and the capacity to switch between a charming exterior and a calculating interior, making her an effective observer and infiltrator. She does not undergo a major personal transformation but remains a consistent, enigmatic presence within the Tomodachi Game's inner workings.