Kageyama is the enigmatic butler and personal chauffeur to Reiko Houshou, the wealthy heiress who works as a police detective. He is a man in his mid-thirties who is typically seen in a perfectly pressed dark suit and silver-rimmed glasses, projecting an image of calm professionalism and impeccable service. Beneath this reserved exterior, however, lies a sharp and often cutting personality. While he always uses polite and formal language with his employer, his words are frequently laced with sarcasm and blunt criticism, earning him a reputation as a suspicious or questionable butler in the eyes of Reiko, who often finds herself the target of his biting remarks.

The true background of Kageyama is shrouded in mystery, as he never shares details about his past or the reasons he chose to become a butler. He has only hinted at his childhood dreams, mentioning that he once aspired to be either a professional baseball player or a detective. Despite the lack of trust Reiko places in him as a servant, she consistently turns to him for help with the complex and baffling criminal cases she encounters. In his role, Kageyama acts as an armchair detective, possessing exceptional deductive skills that allow him to solve even the most difficult mysteries with ease. He typically does this from the comfort of his own home, simply by listening to Reiko narrate the details of a case over dinner, seeing no need to visit crime scenes or gather physical evidence as he considers that to be the responsibility of the police. Only when he believes there is a risk of a second crime will he reluctantly accompany Reiko to an active scene.

Kageyama is defined by an unshakeable calm and a preference for logical reasoning over material investigation. He is proud of his abilities in both deduction and baseball, fields in which he possesses a surprisingly vast knowledge. He often uses the dinner table as his stage, waiting for Reiko to lay out the facts of a case before deconstructing her flawed reasoning and pointing out everything she has missed. After this ritual, he presents a brilliant solution that leaves his employer both amazed and frustrated. While he is not a field agent, Kageyama is not entirely defenseless; he carries a special telescopic baton for self-defense when facing criminals directly. His primary relationship is with Reiko, whom he serves daily while simultaneously challenging her with his sharp tongue. He shows a similar, casually irreverent attitude towards Reiko's father, Kiyotaro Houshou, often making subtle jokes at his expense as well. Driven by a hidden past and a desire to solve puzzles purely for the intellectual challenge, Kageyama remains a static but compelling figure whose development is less about personal change and more about the gradual, but never complete, revelation of his mysterious character.