Sora Kasugano is the fraternal twin sister of Haruka Kasugano. She has a pale complexion, long silver hair typically in outdoor twintails, light gray eyes, and a slim, doll-like frame standing 152 cm tall. Chronic frail health since birth led to frequent hospitalizations, resulting in a delicate constitution and limited independence. Her reclusive, withdrawn personality features laziness, stubbornness, and minimal interest in anyone beyond Haruka. She depends on him obsessively, refusing school without him and spending days indoors browsing the internet or snacking. She clings to a stuffed rabbit doll from her deceased mother, embodying her emotional fragility. Intense jealousy consumes her toward girls close to Haruka, especially Nao Yorihime after witnessing their childhood sexual encounter. This sparks hostility, though she sometimes reconciles with Nao. After their parents died in a car accident, Sora and Haruka moved to their grandfather's rural home. The trauma deepened her reliance on Haruka, amplified by a shared childhood kiss that ignited romantic fixation. Her route explores incestuous attraction: she initiates intimacy during a fever confession, beginning a sexual relationship. When friends discover them and Haruka rejects her over societal shame, she attempts drowning suicide. Haruka rescues her but nearly drowns due to being unable to swim; Sora saves him upon hearing his plea, cementing their bond. They later relocate to Northern Europe while keeping friends. In non-Sora routes like Kazuha, Akira, or Nao, her development diverges. She accepts Haruka's other relationships, growing socially adaptable—befriending Akira Amatsume and helping with cooking. Her animosity toward Nao fades in Nao's route after Nao saves her rabbit doll from fire. Adaptations vary: the anime exaggerates negative traits, showing obsessive acts like typing "I hate Nao" repeatedly and violent clashes with Haruka. The visual novel depicts Kozue Kuranaga eventually accepting the twins' relationship, while the anime leaves Kozue shattered. Anime-only scenes heighten drama, such as Sora witnessing Haruka's intimacy with other heroines.