Maruo Hyakki is a supporting character in Young Black Jack. He was formerly a prominent professor at Teito University, highly regarded in the medical field. His life changed dramatically after a severe car accident that resulted in the amputation of all four of his limbs. Despite this life-altering injury, Hyakki retained his extensive knowledge of medicine, particularly in the area of upper extremity prosthetics. His primary motivation is to regain a degree of independence and function by developing and building prosthetic limbs that can replace those he lost, a goal that drives much of his personal and professional efforts.

In terms of personality, Hyakki is determined and resourceful, channeling his expertise into a specialized area of prosthetic surgery. He does not allow his physical limitations to define him, and his focus on practical solutions reflects a pragmatic and resilient character. His role in the story is that of a skilled surgeon and professor who becomes a source of specialized medical knowledge, particularly when prosthetic devices are needed. He interacts with the protagonist, Kuroo Hazama, during story arcs that involve complex surgical cases and ethical dilemmas.

Hyakki's personal relationships include a romantic involvement with a woman named Mio, and his surgical work extends to treating a young boy named Roro, for whom he creates prosthetic limbs. These connections highlight his compassionate side and his willingness to apply his skills to help others facing physical challenges. Over the course of his appearance in the series, Hyakki undergoes a modest but meaningful development as he adapts to his new reality, reaffirms his commitment to medicine, and finds personal fulfillment through both his relationship and his work. His notable abilities lie in his deep understanding of prosthetic design and surgical techniques, as well as his capacity to innovate under difficult circumstances.