Koutarou Kakinouchi, sometimes spelled Kotarou or Kotaro, is the wealthy heir of the Kakinouchi conglomerate and a classmate of Mayura Daidouji at her school. His affluent background affords him a comfortable life, and he carries himself with the confidence expected of a young man from a prominent family.

In terms of personality, he is notably flirtatious, eagerly pursuing nearly every pretty girl he encounters and cultivating a reputation as a ladies' man. Beneath this playboy demeanor lies a sharp intuition that allows him to perceive details others might overlook. While he can come across as a spoiled rich boy, his presence is rarely malicious; instead, he adds a lighthearted and occasionally self-serving energy to the group.

His role in the story is that of a supporting figure. In the anime he appears in a limited number of episodes, spending time with Loki and his companions. The manga expands his role considerably, where he proves to be a skilled detective on par with Loki himself and becomes a valued friend. In the anime adaptation, his interactions mostly revolve around hanging out with the main cast, and he also has several encounters with the enigmatic Hel.

Koutarou’s key relationship is with Loki, who in the manga nicknames him Kou and comes to consider him a genuine ally. He is a classmate and acquaintance of Mayura, and he interacts with other members of the detective agency as the series progresses. There is no significant arc of personal transformation documented for the anime version; his character remains a charming, intuitive, and occasionally unpredictable presence who reinforces the series' mix of mundane school life and supernatural intrigue. His most notable ability is his perceptive instinct, which in the original manga makes him a capable detective, while even in the anime his quick wits and social confidence mark him as more than just a background figure.