Mitsuki Ooshiro is a tall high school student distinguished by her auburn or light orange hair, typically styled to neck length, striking red eyes, and a pale complexion. Her significant height fuels deep insecurity and shyness, frequently manifesting as blushing and a perpetually worried expression. She consistently wears her school uniform: a white dress shirt fastened with a red bow, a light orange sweater vest, a black or blue mini skirt, white stockings, and brown loafers.

Childhood friends with Reina Aharen since elementary school, Mitsuki's social anxiety initially kept her observing Reina from afar. A pivotal moment occurred when Reina called her "cute," intensifying Mitsuki's shyness around her. During their first year of high school, being in separate classes allowed Mitsuki to maintain her distant observation. They became classmates in their second year, finally offering Mitsuki opportunities for direct interaction.

Characterized by extreme nervousness, emotional vulnerability, and a tendency to cry easily, Mitsuki harbors a powerful protective instinct toward Reina. This instinct initially led her to perceive Raido Matsuboshi as a threatening "bad guy" due to his stern features. She resorted to covertly stalking Reina and Raido while declaring her intent to "protect" Reina. Witnessing their closeness sparked jealousy, though Mitsuki gradually acknowledged Raido's genuine kindness after Reina defended him. This recognition enabled her to shift from distant observation to active engagement, approaching Raido directly.

Her development culminated upon learning of Reina's romantic confession to Raido, prompting Mitsuki to challenge him to an Othello match. She stipulated that Raido must distance himself unless his feelings matched Reina's. Raido's victory confirmed his sincerity, leading Mitsuki to accept their relationship and transition into a supportive role within their friend group. She maintains friendships, regularly sharing lunch with classmates like Riku Tamanaha.

Mitsuki possesses notable physical strength and athleticism, honed through activities like boxing and muscle training. Her dexterity enables surprising feats such as cutting rolled mats with a sword or catching bees with chopsticks. She demonstrates skill in classroom concealment, though Reina consistently detects her hiding spots. Her parents' ownership of a barbershop equipped her with skills in hair cutting, makeup application, and massage, which she occasionally performs for Reina.

Her name "Mitsuki" (蜜月) translates to "honeymoon," combining "honey" (蜜) and "moon/month" (月), while "Oshiro" (大城) means "large castle."