Akane Mizuhara is the younger sister of Ryo Mizuhara. Following their parents' unexplained death years before the main events, she lives with Ryo. Orphaned young, she developed profound emotional reliance on her brother, viewing him as her primary source of stability and happiness, a dependency central to her character.

She attends the same school as Ryo, enrolled in the 10th grade. Gentle-minded, she observes surrounding dynamics with quiet perception. Despite upheaval from Shizuka Masuda's sudden introduction as Ryo's government-arranged fiancée, Akane offers no overt objection. Her acceptance is passive, stemming from fear Ryo might neglect her after marriage rather than active support. A sisterly bond forms with Shizuka, enabling Akane to intuitively grasp Shizuka's emotional struggles.

This deep reliance surfaces starkly when Akane falls ill. Feverish dreams reveal past memories and anxieties about Ryo abandoning her for marital responsibilities. This fear, coupled with her expressed happiness solely in Ryo's presence, suggests complex feelings extending beyond typical sibling affection. Subtle implications are further suggested during a group ski trip where she actively participates in a competition among female characters for the role of Ryo's bride.

Akane generally maintains a supportive, observational role within the group dynamics involving Ryo, Shizuka, and their friends. She narrates key developments, such as the dissolution of Ryo's temporary dating relationship with classmate Emiho Mutsu, noting their reversion to pre-dating normalcy without formal breakup. Her consistently amicable relationship with Shizuka serves as an emotional barometer for Shizuka's experiences while Akane navigates her own insecurities about her place in Ryo's life amidst the forced marriage arrangement.