Hotaru Mizushina is one of the two main protagonists in her story, serving as the childhood friend and primary emotional anchor for her counterpart, Yuma Okazaki. She is a high school student known throughout her school as a petite and strikingly beautiful girl, though her delicate appearance belies a more complex and contradictory nature.

Outwardly, Hotaru presents herself as confident, extroverted, and at times, sexually adventurous. She is a veteran of romantic relationships, having dated multiple boys since middle school, which has earned her a reputation among some classmates as being promiscuous, though she generally pays such gossip little mind. This public persona, however, masks a deep-seated loneliness and an intense emotional dependency on Yuma, which forms the core of her character. Her early school years were marked by bullying from boys, an experience from which Yuma once rescued her. This event forged a powerful bond, and Hotaru has since looked to Yuma as her primary source of security and validation.

The central conflict for Hotaru stems from her inability to directly express her romantic feelings for Yuma. To cope with her unspoken desires and the fear of losing her best friend's exclusive attention, she engages in a pattern of self-destructive behavior. She dates boys frequently but without genuine emotional attachment, using these relationships as a distraction or a way to provoke a reaction from Yuma. Her most significant relationship is with her boyfriend, Fujiwara, which is depicted as dysfunctional and transactional. Their arrangement is built on a pact to satisfy each other's physical needs, but it lacks authentic love, and Fujiwara responds with aggressive behavior when Hotaru does not comply with his expectations.

Instead of confessing her true feelings, Hotaru initiates physical intimacy with Yuma under the guise of helping her practice for her own boyfriend, Takeda. These encounters, ranging from teasing touches to deep kisses, are emotionally charged and manipulative. They serve a dual purpose: they allow Hotaru to express her genuine desire while maintaining a layer of plausible deniability. Her jealousy and possessiveness become most apparent when Yuma shares news of her romantic progress with Takeda, as Hotaru often responds immediately with a more passionate kiss, marking her territory and reasserting her primacy in Yuma's life.

Throughout the narrative, Hotaru's motivations are shown to be rooted in a profound need to be needed. She is deeply afraid of being abandoned or replaced, a fear that drives her emotional manipulation. Her behavior is unpredictable, with her motives shifting between a playful tease and a desperate, lonely girl. This complexity makes her a challenging figure for Yuma to understand, as Hotaru rarely states her feelings plainly. Her relationship with Yuma evolves from a seemingly one-sided teasing game into a reciprocal acknowledgment of their mutual romantic attachment. Ultimately, her actions are a cry for Yuma to see through her façade and to recognize that their connection has long surpassed the boundaries of ordinary friendship. Beyond her emotional and social skills, her role in the story is defined by her ability to initiate and escalate the central drama, pushing Yuma to confront her own confused feelings in response to Hotaru's advances.