Mitsuki Nase, born November 1st, is the youngest member of the prestigious Nase clan of Spirit World Warriors who monitor supernatural entities called youmu. Confined to the Nase residence during her formative years due to family duties, she developed a detached and emotionally reserved demeanor shaped by her older sister Izumi's teachings that obligations supersede personal connections, resulting in limited childhood peer interactions.

Physically, she has long black hair, red eyes, and a slim yet curvy figure standing 165 cm tall. She typically wears her high school uniform: a yale-blue sailor-style top with a red ribbon, a light blue pleated skirt, calf-length black socks, and black-and-white shoes. She is frequently depicted consuming sweets like lollipops or ice cream.

As a second-year high school student and president of the Literary Club, she spends significant time with members Akihito Kanbara and her older brother Hiroomi Nase. Her personality blends a blunt, near-sadistic exterior with moments of genuine care, often masking deeper emotions. She engages in teasing banter with Akihito, frequently calling him a "pervert" due to his glasses fetish, yet demonstrates unwavering concern for his safety, especially regarding crises tied to his half-youmu nature.

Her duties include overseeing Akihito as a potential threat due to the powerful youmu "Beyond the Boundary" within him. Initially wary of the blood-manipulating Spirit Warrior Mirai Kuriyama, Mitsuki advises Akihito to avoid her per clan orders. She gradually warms to Mirai, facilitating her integration into the Literary Club and assisting her financially by referring her to a part-time job after Mirai loses her Spirit Warrior license. This culminates in Mitsuki attending a lantern festival with Mirai, marking her first participatory social outing.

During major conflicts, her role intensifies. She aids in combating the Hollow Shadow youmu and later coordinates with her siblings during the Calm—a phenomenon agitating youmu—to contain Akihito's rampage. When Mirai sacrifices herself to save Akihito, Mitsuki's grief manifests subtly. Months later, she collaborates with Hiroomi and Izumi against Spirit Warrior Miroku Fujima, who exposes Izumi's secret fusion with a youmu. This humiliates Izumi into fleeing, transferring clan leadership to Hiroomi and Mitsuki. Following Beyond the Boundary's defeat, Mitsuki tearfully reunites with a resurrected Mirai.

Mitsuki wields the Nase family's "Glaciate Barriers" for defensive containment or offensive attacks against youmu. Though less proficient than her siblings, she compensates using her pet rodent-like youmu, Yakiimo, in combat. Her abilities wane temporarily during the Calm but later restore.

Key relationships define her character:
- With Akihito, she balances duty and friendship, serving as his rational advisor while sharing comedic exchanges as the "tsukkomi" (straight person).
- Her dynamic with Hiroomi involves frequent bickering over his sister-complex tendencies, yet they share mutual trust and protective instincts during battles.
- Her bond with Mirai evolves from cautious neutrality to heartfelt camaraderie, exemplified when Mitsuki hugs Mirai while crying upon her return.

In the spin-off ONA *Idol Trial!*, Mitsuki participates in a comedic idol group that judges wrongdoers.