Tsubasa, whose true identity is Tsuyoshi, is a male student blackmailed by his twin sister into impersonating her at Kashihara Girls' Academy. This switch occurred because she wished to attend his originally intended all-boys school to pursue a crush on a male idol student there. She used compromising photos of Tsuyoshi crossdressing to enforce the ruse.

He possesses short gray hair, blue eyes, and a naturally androgynous appearance enabling him to pass as female, a situation causing him occasional discomfort. A distinguishing feature from his twin is his pierced ears. Though initially resistant, he adapts to attending the all-girls school, navigating the challenges of maintaining his false identity, including wearing restrictive uniforms and managing social interactions.

Within his friend group—Rikka (an alien), Chiyo (a former ninja), and Sekine (a psychic)—Tsuyoshi's secret remains partially protected. Sekine's telepathy only works on individuals of the same gender, preventing her from detecting his thoughts while he presents as female. This limitation becomes evident when Rikka's alien technology temporarily transforms the school into an all-male environment, altering Sekine's telepathic range. Tsuyoshi harbors a romantic preference for older women, diminishing potential interest in classmates.

His relationship with his twin sister is contentious, marked by her manipulation. She undermines his social standing by badmouthing him to peers, sells gifts intended for him, and shows general disregard for his well-being. Throughout incidents—including a failed poisoning attempt by Chiyo's brother that Rikka accidentally thwarted, and a cultural festival revealing Chiyo's ninja skills—Tsuyoshi remains entangled in the group's chaotic dynamic while guarding his secret.

A significant development arises when Rikka briefly uses alien technology to physically transform Tsuyoshi into a biological girl, intensifying his internal conflict about gender identity and appearance. This incident, coupled with mounting pressures, culminates in Tsuyoshi revealing his true identity to his friends. The disclosure meets acceptance, particularly from Chiyo, who prioritizes their friendship over the deception. The group's bond persists despite the revealed secrets, though Tsuyoshi continues the charade in broader social contexts due to his sister's coercion.