Hisashi Sakuma serves as a supporting character in the Chaos;Child anime. He is introduced as a doctor working at the Aoba Clinic, presenting a professional and unassuming front in his medical role.

As the narrative progresses, Sakuma is revealed to be the true mastermind behind the revived New Generation Madness murders. His motivations and actions are deeply intertwined with a larger conspiracy, as he is affiliated with a powerful organization responsible for many of the strange events in the Science Adventure series. This affiliation drives his secret experiments and schemes.

Sakuma maintains a significant conspiratorial relationship with Serika Onoe, working together with her to carry out the series of brutal killings that form the central mystery of the plot. His past includes conducting unethical experiments at a major hospital, which has a direct connection to the origins of the phenomenon at the heart of the story.

In his role in the story, Sakuma represents the hidden, calculating antagonist who has been manipulating events from the beginning. His plans directly involve the protagonist, Takuru Miyashiro, and Serika Onoe. At a crucial point, he challenges the two to meet him at the place where the entire incident began, setting the stage for the final confrontation. While not a front-line fighter, his abilities lie in his intellect, his scientific knowledge, his strategic manipulation of others, and the authority granted by his powerful benefactors.