Mitsuki Ōshiro is Reina Aharen's childhood friend since elementary school. Her tall stature fuels significant insecurity and shyness, manifesting as frequent blushing and a perpetually worried expression. This height complex sharply contrasts with Reina's petite frame, intensifying Mitsuki's awkwardness and self-consciousness. Consequently, she often observed Reina from a distance rather than interacting directly, despite strong protective instincts.

During elementary school, Reina once called Mitsuki "cute," an incident that deepened her shyness and left a lasting impact, reinforcing hesitance to engage openly. Mitsuki's parents own a barbershop, equipping her with practical skills in haircutting, makeup application, and massage; she occasionally cuts Reina's hair, leveraging this family background. Mitsuki also possesses notable physical abilities, including proficiency in boxing and muscle training, alongside surprising feats like slicing rolled mats with a sword or catching bees with chopsticks. Her athleticism extends to hiding effectively in classroom environments, though Reina typically locates her.

In high school, Mitsuki and Reina were initially in different first-year classes, enabling her continued distant observation. When placed together in their second year, Mitsuki seized the chance for more direct interaction. Around this time, she encountered Shōta Raidō, Reina's seatmate. Mistaking Raidō's stern expression for malice, she perceived him as a threat to Reina and began spying on them. Protective instincts manifested as stalking behavior, driven by jealousy and suspicion that Raidō might supplant her role as Reina's primary supporter. Discovered by Raidō during one observation, Mitsuki confronted him, accusing him of deceiving Reina.

Reina intervened, reassuring Mitsuki of Raidō's kindness and persuading her to lower her guard. Though initially unconvinced, Mitsuki observed positive changes in Reina's demeanor around Raidō, softening her stance. To test Raidō's sincerity, Mitsuki challenged him to an Othello match—a game she had never lost, even against Reina—stipulating that if Raidō lost, he must distance himself from Reina. Raidō's determined victory and genuine affection for Reina convinced Mitsuki of his trustworthy nature, leading her to accept him.

Subsequently, Mitsuki gradually integrated into their social circle, joining group activities like shared lunches. She formed a friendship with Riku Tamanaha, a transfer student and Reina's elementary classmate, though Riku did not initially recall Mitsuki due to frequent relocations. Mitsuki's journey reflects a shift from distant, anxious observer to involved friend, navigating her insecurities while maintaining her protective and caring nature towards those closest to her.