Mari Setagaya is a hybrid supernatural entity—half-vampire and half-succubus—exhibiting distinct physiological and behavioral traits. She openly displays her non-human nature, contrasting traditional vampires through her naivety and lack of restraint. Her vampire lineage compels feeding on human life energy via bodily fluids, with blood as the most efficient source. However, she finds blood intolerable and sustains herself on sweat, saliva, or semen. Sweat or saliva consumption typically induces immediate unconsciousness, while semen provides nourishment without side effects, becoming her preferred sustenance.

Physically, she has red hair, red eyes, and stands 158 cm tall, with a 91 cm bust, 59 cm waist, and 88 cm hips. Functional bat wings serve as proof of her supernatural identity. Her succubus heritage allows sensing a partner's subconscious desires and altering her body accordingly—such as enlarging her bust—triggering a drastic personality shift from naive impulsiveness to lascivious seduction. Despite this transformation, she remains consistently outspoken and prone to verbal overcommitment.

Her development centers on her relationship with Kanzaki. She initially lures him with a forged love letter to feed, but after failing to induce unconsciousness, reveals her identity and dietary limits. Kanzaki proposes semen as an alternative; she impulsively accepts due to hunger, establishing a recurring feeding arrangement that evolves into mutual attachment. During encounters, his desires activate her succubus abilities, prompting physical changes and heightened sexuality. Despite enduring a non-consensual encounter with others—witnessed by Kanzaki—she maintains loyalty to him and prioritizes their bond afterward.