Aqua is the male protagonist and one of the titular characters of Oshi no Ko. In his previous life, he was known as Gorou Amamiya, a medical doctor who was murdered by a stalker of one of his patients, Ai Hoshino, and reincarnated as the latter's child. meanwhile Ruby  is the female protagonist and one of the titular characters of Oshi no Ko. In her previous life, she was known as Sarina Tendouji, a terminally ill patient who was cared for by Gorou Amamiya.