Kōta Koga is the younger of a pair of twin brothers. In contrast to his twin Rakuta, who is calm and measured, Kōta is quick-tempered and tends to act without fully considering the consequences. This impulsiveness often gets him into trouble, but it also propels the group into action when a more cautious approach might delay progress. He attends the same school as the other main characters and frequently finds himself in the midst of the mysterious cases that arise in Kyuto.

Kōta's primary motivation stems from his romantic infatuation with a girl named Yaya. However, he frequently observes Yaya directing her attention toward the protagonist, Mayuki, which provokes strong feelings of jealousy in Kōta. This envy is a recurring source of conflict for him, as it fuels a rivalry with Mayuki even as Kōta continues to accompany the group on their investigations. Despite his resentment, he does not hesitate to join his friends when danger calls, and he works alongside them to solve puzzles and confront threats.

His key relationships include his twin brother Rakuta, who often acts as a foil to Kōta's hotheaded nature. The brothers share a bond but have distinctly different approaches to problems. Kōta also maintains a tense dynamic with Mayuki, whom he sees as a rival for Yaya's affection, yet he still cooperates with him when necessary. His connection to Yaya is one of unrequited affection that influences many of his decisions.

In terms of character development, Kōta remains largely consistent throughout the series, though his experiences in the various cases sometimes temper his impulsiveness and teach him the value of teamwork. Notable abilities are not explicitly supernatural; instead, Kōta relies on his physical energy and readiness to leap into action, making him a useful asset in situations that require bravery or a direct approach. He does not possess the deductive brilliance or psychic powers of Mayuki, but his willingness to act can turn the tide when planning alone is not enough.