Hazumu Osaragi, born male in the fictional Kashima ward near Tokyo, displayed distinctly feminine traits from early childhood, including sensitivity, a gentle demeanor, and an interest in traditionally feminine activities like floriculture. Their mother often dressed them in girls' clothing, which they enjoyed, leading others to frequently mistake them for a girl due to their appearance and mannerisms. Hazumu shared a close childhood bond with Tomari Kurusu, who protected them from bullies. During this time, Hazumu expressed a desire to become Tomari's bride, indicating an early, subconscious alignment with a female identity. Academically average, Hazumu joined the Gardening Club in high school, demonstrating extensive horticultural knowledge. Pressured by friends Tomari and Asuta Soro in their second year, Hazumu confessed feelings to classmate Yasuna Kamiizumi, who rejected them due to her inability to see males clearly. Distraught, Hazumu retreated to Mount Kashima, where an alien spacecraft crash-landed, causing fatal injuries.

Alien Hitoshi Sora revived Hazumu using "female life grains" to rectify the accident, resulting in a complete, irreversible biological sex change down to the DNA level. This transformation was broadcast globally, leading to widespread acceptance of Hazumu's new identity. Physically, Hazumu now possessed a slender, underweight but proportionate teenage girl's body with a noticeable bust (explicitly noted as "32 C" in the manga), reddish-brown hair in a pixie cut, maroon eyes, and even skin tone. While retaining core personality traits like kindness, selflessness, and indecisiveness, Hazumu gradually adopted more feminine mannerisms and expressions, though they continued using the masculine Japanese pronoun "boku" for self-reference. Initial confusion arose over female-specific social norms, such as clothing choices (particularly bras) and bathroom etiquette, with adaptation occurring faster in the manga than in the anime, where Tomari provided extensive guidance. Hazumu's mother enthusiastically curated an ultra-feminine, kawaii, and mildly alluring wardrobe, including the school uniform (a blouse with a bow tie and short red jumper dress) and occasional Sweet Lolita fashion.

Hazumu's development centered on navigating romantic relationships within a love triangle involving Yasuna and Tomari. Yasuna, who could not see males clearly prior to Hazumu's transformation, immediately professed love post-change, drawn to Hazumu's visibility and inherent femininity. Tomari, Hazumu's childhood friend, struggled initially but gradually acknowledged her own romantic feelings, intensifying Hazumu's indecisiveness and conflict between the two girls. Key narrative differences emerged between the manga and anime. In the manga, Hazumu faced a life-threatening condition one month after revival, requiring a "life grains" donation from someone close. After Tomari risked her life to save Hazumu during a fall, Hazumu chose her over Yasuna, reciprocating Tomari's feelings. The anime omitted this crisis; Hazumu chose Yasuna to help stabilize her deteriorating vision (which now affected females). After Yasuna's vision was fully restored, she broke off the relationship to assert independence. The OVA sequel resolved this with Hazumu confessing to Tomari, culminating in a Christmas marriage ceremony officiated by Hitoshi.

Hazumu's story arc reflects a journey toward self-acceptance and emotional resolution. Core aspects of their identity—loyalty to friends, love for nature (especially gardening), and a nurturing disposition—remained consistent despite the physical change. The lack of desire to revert to male, coupled with pre-transformation behaviors, strongly implies an underlying transgender identity validated by the transformation. Hazumu's greatest happiness consistently stemmed from being surrounded by loved ones, ultimately finding fulfillment in a committed relationship with Tomari.