Koya Inukai is a male student in Class 2-A at Holy Cross High School, seated beside protagonist Riku Aoba. Standing 182 cm tall, he possesses gray shoulder-length hair and gray eyes. His altered school uniform contributes to an initial impression of a loner or delinquent. Born on August 8 with blood type B, Inukai's intimidating appearance belies a notably kind and warm personality. He remains reserved about his personal life, preferring to express himself through actions like caring for small animals—including befriending a kitten—and demonstrating a talent for painting that reveals a creative, gentle nature.

This reserved demeanor often leads to misunderstandings, exemplified when rumors spread following an alleged fight with thugs that resulted in a brief arrest. The incident caused his absence from school, further fueling perceptions of rebelliousness. However, it also sparked deeper curiosity about his true motivations, particularly from Riku. Inukai gradually forms a meaningful yet unacknowledged friendship with Riku. The depth of his unspoken loyalty and emotional investment becomes starkly evident when he is visibly shocked upon learning of Riku's impending transfer on the day of the closing ceremony.