Mitsuki is a young woman of average height with a slim yet curvy build, fair skin, and dark ruby-colored eyes. Like many members of the Nase family, she has long black hair, which she typically wears straight down her back with loose fringes framing her forehead. On occasion, such as during the festival, she styles it in a side ponytail, though she retains her bangs regardless of hairstyle.

A notable aspect of her design is her frequent habit of carrying or eating a lollipop or popsicle, which has become her signature look.

Her standard outfit is her school uniform: a sailor-style top in yale blue with white and blue cuffs, three buttons on either side, and a long striped collar tied with a red ribbon. This is paired with a light blue pleated skirt trimmed with white, calf-length black socks, and black lace-up shoes with white accents.

Mitsuki is characterized by her blunt manner of speaking and a tendency to act with a sharp, almost sadistic edge. This cold and disciplined attitude stems largely from the influence of her cousin Izumi Nase, who instilled in her the belief that Spirit World Warriors must devote themselves entirely to their duties. Beneath this reserved exterior, however, Mitsuki is capable of showing a wider range of emotions, including anger, sadness, and joy.

As president of the Literature Club, she often spends time with Akihito Kanbara, forming part of the group’s comedic dynamic by teasing him particularly about his obsession with glasses. Outside her duties, she has a fondness for sweets and ice cream, a recurring trait in her characterization.

Unlike many Spirit World Warriors, Mitsuki demonstrates a more nuanced view of Youmu, believing that some can coexist peacefully with humans without posing a threat. This perspective reflects her capacity for empathy, despite her otherwise stern outlook.

During her childhood, Mitsuki was raised under the strict guidance of Izumi Nase, who emphasized self-reliance and the duty of Spirit World Warriors above all else. As a result, Mitsuki spent much of her early life within the confines of the Nase residence, with limited opportunities to interact with others outside her family. This upbringing fostered a reserved and detached outlook, making it difficult for her to form friendships.

By the time of the series, Mitsuki’s social interactions are largely limited to members of the school’s Literature Club, particularly Akihito Kanbara and her brother Hiroomi Nase, with whom she gradually begins to open up.

Glaciate Barriers
Like her siblings, Mitsuki is capable of generating Glaciate Barriers, specialized constructs that can serve both defensive and offensive purposes against Youmu. While effective, she tends to use them sparingly in battle, preferring to rely on her familiar, Yakiimo, for direct combat.

Although competent, Mitsuki is not considered as skilled or powerful as her siblings Hiroomi and Izumi, and often requires their assistance when faced with stronger opponents.

Akihito Kanbara
Mitsuki frequently spends time with Akihito in the Literature Club room, where their dynamic often takes the form of lighthearted banter, with Mitsuki serving as the tsukkomi to his antics. Officially, she accompanies him as part of her duty to monitor and safeguard him due to his Half-Youmu nature. Despite her sharp remarks, Mitsuki genuinely cares for Akihito as a friend and has shown deep concern for his well-being, even to the point of tears when he is seriously injured. Throughout the series, she often provides him with practical and rational advice.

Mirai Kuriyama
Initially cautious of Mirai due to warnings from her family, Mitsuki gradually grows closer to her and forms a strong bond. Their relationship develops through shared experiences, and by the events of Beyond the Boundary Movie: I’ll Be Here – Future, Mitsuki openly expresses her care for Mirai. She scolds Akihito for keeping Mirai’s past a secret, and in the film’s conclusion, she embraces Mirai in tears, signifying their close friendship.

Hiroomi Nase
Mitsuki’s relationship with her older brother Hiroomi is defined by a mix of affection and exasperation. The two often tease and bicker with one another, especially over Hiroomi’s sister complex, which Mitsuki finds embarrassing or distasteful. Nevertheless, their quarrels are short-lived, and they share a strong familial bond. Hiroomi often steps into the role of a protective older brother when necessary, while Mitsuki, in turn, expresses gratitude and care for him.

The given name Mitsuki (美月) combines mi (美), meaning “beautiful,” and tsuki (月), meaning “moon.” Her surname Nase (名瀬) is composed of na (名), meaning “name,” “noted,” or “distinguished,” and se (瀬), meaning “swift current” or “rapids.” The surname can be interpreted as “renowned position,” reflecting the prestige of the Nase family within the community of Spirit World Warriors.