Kazuomi Ōga is a third-year student at Aoba High School who becomes close classmates with Arata Kaizaki. He consistently earns the highest academic scores among the male students in his grade, frequently landing him the role of class representative. Despite his academic strengths, he struggles with athletic activities and starts off remarkably oblivious to romantic matters, completely unaware of Rena Kariu's long-standing feelings for him. His family's financial difficulties drive his academic diligence to secure scholarships and avoid burdening them with educational costs, also forcing him to work a part-time job outside school hours.

He has light brown hair and brown eyes. During high school, he wears a hand-me-down uniform from his older brother: a white shirt, black tie, dark gray trousers, and a loose-fitting light gray full-sleeve sweater. A noticeable piercing in his left ear matches one worn by Rena Kariu, originally done by his brother. Multiple characters, including Arata Kaizaki and An Onoya, comment favorably on his attractive physique and appearance.

Kazuomi deeply respects his older brother, Takaomi Ōga, who previously supported the family financially before becoming a NEET. The ear piercing serves as a significant memento of his brother's happier, more productive period. Kazuomi accepts the shift in family responsibility without resentment, believing his brother deserves time to recover. His family background includes his mother leaving when he was three, an event that led to social rejection. His father experienced unemployment due to the separation shock but eventually resumed work as an office employee.

His relationship with Arata Kaizaki is warm and mutually supportive. Kazuomi extensively assists Kaizaki with remedial academic tests and frequently seeks his advice, perceiving Kaizaki as romantically experienced despite Kaizaki's objections. Kaizaki plays a pivotal role in helping Kazuomi recognize his own romantic feelings for Rena Kariu, whom he has known since middle school. Kazuomi's initial obliviousness to Rena's affection gradually gives way to self-awareness, culminating in a mutual confession during a summer festival. Their relationship evolves into a committed partnership, though Kazuomi occasionally prioritizes work over dates due to his part-time job, causing temporary tensions.

As graduation approaches, Kazuomi faces decisions about university, weighing financial constraints against his desire to attend the same institution as Rena. Concerns about following his brother's path add complexity. Ultimately, following guidance from Kaizaki and Chizuru Hishiro, he opts to enroll locally at Aoba University. His character development includes overcoming communication gaps with Rena regarding his educational plans and navigating familial challenges. Kazuomi and Rena eventually marry, with Rena taking his surname, Ōga.