Nagi Umino is the main male protagonist of A Couple of Cuckoos. He is a second-year student at Megurogawa Academy, where he consistently ranks second in his grade. Nagi is the biological son of a wealthy hotel tycoon, but due to a hospital mix-up shortly after birth, he was raised by a working-class family, the Uminos, who run a small diner. This upbringing instilled in him a strong sense of responsibility, domestic competence, and a down-to-earth attitude. He is studious and highly disciplined, setting academic excellence as a priority. A significant motivation early in the story is his desire to confess to his classmate and academic rival, Hiro Segawa, a goal he ties to finally surpassing her in the school rankings.

Nagi’s personality is characterized by a serious, diligent demeanor mixed with a tendency to become flustered when confronted with the romantic chaos that unfolds around him. He is principled, loyal to his adoptive family, and possesses a strong will, often refusing to be pushed around by the whims of the wealthy Amano family. Despite his levelheadedness in studies and household chores, he is notably naive and easily overwhelmed when dealing with the romantic advances of the girls in his life. He is also physically capable; having been influenced by his adoptive parents, who were former bullies, he is a competent fighter, occasionally displaying surprising strength.

His role in the story is that of the central figure caught in an arranged marriage with Erika Amano, the girl with whom he was switched at birth. The narrative follows his journey as he navigates the forced cohabitation and engagement, gradually moving from outright resistance to developing genuine, complicated feelings for Erika while also wrestling with his long-standing crush on Hiro and the protective, evolving bond with his adoptive sister, Sachi Umino. Nagi’s arc involves reconciling his identity between his biological and adoptive families, learning to understand his own emotions beyond academic achievement, and becoming more open to vulnerability. Notable abilities include his sharp intellect, strong domestic skills (cooking and cleaning are second nature to him), and his unexpected physical prowess in combat situations.