Kakinouchi Kotaru is a supporting character in the anime Mythical Detective Loki Ragnarok, serving as one of the few prominent human figures in a story largely populated by Norse gods in human form. He is introduced as a classmate of Mayura Daidouji, the series' enthusiastic human heroine, and is established from the outset as a young man of significant wealth and social standing.

Kotaru is the heir to the vast Kakinouchi conglomerate, a position that has shaped his outward personality considerably. He carries himself with an air of self-assurance and pride, often appearing arrogant or as a spoiled rich boy who thinks highly of himself and his family's many business holdings. This manifests in a flirtatious demeanor, and he is known to be a bit of a womanizer, frequently charming female characters around him with little apparent effort. This confident and somewhat superficial exterior can easily lead others to misinterpret his intentions, and his attitude has, on occasion, made him a suspect in various incidents or mysteries, though he often remains unconcerned because he is secure in his own innocence.

Beneath this polished and self-centered facade, Kotaru possesses a sharper mind and a more complex character than he initially lets on. He has demonstrated keen senses and a notable level of intelligence, even managing to see through one of the disguises employed by the god Loki, recognizing the detective behind the ruse. This perceptiveness suggests that his frivolous demeanor may be a deliberate choice rather than a lack of depth. Furthermore, despite his egotistical presentation, he has shown a genuine capacity for caring about other people, revealing a more compassionate side that contrasts with his public image. He becomes a regular part of the central group, integrating into the team that forms around the Enjaku Detective Agency.

His role in the story is primarily that of a recurring human ally and occasional source of comic relief, grounded in the mundane world of school and high society that contrasts with the supernatural conflicts of the Norse gods. He adds a dynamic of normalcy and earthly complication to the group. His key relationships are with the main cast, particularly Mayura, whom he knows from school. While the exact nature of his bond with each character varies, his status as a wealthy human often brings different resources and perspectives to the group's adventures. He also has a peculiar tendency to encounter Hel, the goddess of the underworld and Loki's daughter, suggesting an unexplained connection or a recurring narrative thread between the mortal heir and the divine figure.

Kotaru does not undergo a dramatic, transformative development arc. Instead, his progression is one of integration and revelation. He moves from being simply a wealthy, boastful classmate to a trusted, if still occasionally arrogant, member of the team. His development is more about the gradual exposure of his underlying intelligence and loyalty beneath the surface-level traits of a flamboyant rich boy. As a human, Kotaru does not possess any supernatural or magical abilities in the same vein as the gods. His notable abilities are entirely mundane but still effective: sharp observational skills, financial resources from his family's empire, and a charismatic, persuasive presence that he often uses for flirtation but could theoretically apply to other social situations.