Hiderō Koishikawa, also known by the stage name Sonoko Morigaoka, is a fifteen-year-old character from the anime To-Y. He is the cousin of the protagonist, To-Y Fujii, the lead singer of the band GASP. At his young age, Hiderō leads a dual life as a successful idol star, performing publicly under the female persona of Sonoko Morigaoka.

Personality and motivation details for Hiderō are not extensively documented, but his role is defined by a deep, romantic interest in his cousin, To-Y. This attraction is apparent enough to be noticed by others, as Niya, To-Y’s close companion, refers to Hiderō using the Japanese term okama, which characterizes him as an effeminate or gay man. This label highlights the perceived nature of his devotion to To-Y and creates a point of tension with Niya, who positions herself as To-Y’s lover.

Within the story of To-Y, Hiderō's significance is tied directly to this relationship dynamic. He is a key figure in the personal entanglements surrounding To-Y, who must balance the affections of his eccentric friend Niya and his beautiful cousin Hiderō, alongside the pressures of the music industry. As a popular idol, his public status contrasts sharply with the underground, rebellious world of GASP. Regarding his abilities, Hiderō possesses the talent and charisma to have achieved fame as a young idol singer, performing under the Sonoko Morigaoka name. Information about a specific character development arc or other notable abilities for Hiderō Koishikawa is not available in the provided search results.