Billy is a character from Twin Angels BREAK who serves as one of the Four Heavenly Kings, a group he is affiliated with, and also describes himself as a genius. His background involves enrolling as a temporary student at St. Cherine Academy's middle school, where he adopts the alias Shouri Tanaka with the specific goal of uncovering the true identities of the heroines Angel Rose and Angel Sapphire.

In terms of personality, Billy presents a contradictory profile. He is genuinely intelligent in the fields of science and mathematics, possessing a sharp analytical mind for these subjects. However, this expertise is juxtaposed with a notable lack of common sense and overall competence in practical work, leading to a portrayal where his genius is highly specialized and narrow. This is further complicated by the fact that he is a student in the humanities at his university, despite his scientific prowess, and his considerable attributes do not seem to attract attention from women.

Billy's primary motivation is tied to his mission to expose the Twin Angels. His role in the story initially revolves around his infiltration of St. Cherine Academy to achieve this goal. His key relationship is with Mary, the leader of the Four Heavenly Kings. After Billy fails in his mission to identify Angel Rose and Angel Sapphire, Mary expels him from the group. This event becomes a turning point, as his subsequent motivation shifts to seeking revenge against Mary for his dismissal, leading to his return later in the narrative. His notable abilities lie in his scientific and mathematical genius, even if this expertise does not translate into general life competence or success in his espionage duties.