Kazuki Endo is a first-year student and a central figure at Bell Liberty Academy, the prestigious all-boys school that serves as the primary setting for his story. His true name is Kazuki Suzubishi, and he is the heir to the powerful Suzubishi Corporation, the business group that owns the academy. Following the last wishes of his late grandfather, the school's former chairman, Kazuki lives a double life: he is simultaneously a normal student and the acting chairman of the academy's board of directors.

In terms of appearance, Kazuki is a tall young man with light brown hair, which is slightly longer in the front and parted to show his forehead. He has thin, soft-arched eyebrows and distinctive purple-colored eyes. His demeanor is innocuous and perpetually friendly, a gentle exterior that completely masks his abilities as an excellent and capable executive. He possesses a serious side but is generally very kind, and he is known to be highly protective and somewhat jealous when it comes to his close friend, Keita Itō. While he is generally responsible, he is also willing to break rules, such as spending nights off-campus without permission when he deems it necessary.

Kazuki's primary motivation is deeply connected to Keita Itō, the protagonist. Unbeknownst to Keita at first, Kazuki is his childhood friend. In their youth, Kazuki, who Keita called Kazu-nii, made a promise that they would one day attend the same school together. Kazuki never forgot this vow. When the opportunity arose, he used his authority as chairman to admit Keita to Bell Liberty Academy, even though Keita was an average student with no exceptional talents, which was against the school's standard admission policies. His actions are driven by a desire to fulfill their shared promise and to protect Keita, whom he clearly remembers and cares for deeply.

Kazuki's role in the story is multifaceted. He becomes Keita's first and closest friend at the academy, immediately taking the new transfer student under his wing. He is also a member of the school's handicrafts club. However, his secret identity as chairman pulls him into a significant conflict. The Vice Chairman of the school board views Kazuki as a young and incompetent leader and seeks to have him removed from power. This power struggle directly endangers Keita, as the Vice Chairman tries to have him expelled for being an unworthy student. To protect Keita and his own position, Kazuki stakes his role as chairman on the outcome of the school's annual MVP Battle, a competition he must win together with Keita.

The key relationship in Kazuki's life is with Keita Itō. This connection is the emotional core of his character arc. He is a comforting and stable presence for Keita, guiding him through the unfamiliar environment of the elite school. As the story progresses, the mystery of his past with Keita is revealed, transforming him from a kind classmate into a figure who has orchestrated their reunion. He is also protective of Keita in other social situations, such as when he intervenes to take Keita away from the overly aggressive tennis club captain, Yukihiko Naruse, whom Kazuki acknowledges is not a bad person but one who does not give up easily when he likes someone.

Throughout the narrative, Kazuki undergoes development that primarily involves the revelation of his hidden life and his past. He starts as a seemingly perfect and mysterious friend but is gradually revealed to be a young man carrying the heavy responsibility of an entire corporation and school, who is willing to risk it all for a childhood promise. His secret is a major source of drama, but his core personality remains one of devoted protectiveness. Among his notable abilities are his skill as an executive and leader, capable of managing the affairs of a large institution while still a student. He also demonstrates the willingness and resourcefulness to take decisive action, such as borrowing a bicycle to retrieve Keita from an uncomfortable situation.