Masaomi Asahina is the first and eldest of the thirteen Asahina brothers. He works as a pediatrician, a profession that suits his naturally gentle and compassionate nature. Masaomi is kind, patient, and particularly good with children, reflecting a warmth that extends to his entire family. He is generally laid-back and prefers to spend his free time relaxing at home, rarely losing his temper, though when he does get truly angry, his calm demeanor becomes intimidating. Following the death of their father, Masaomi took on a paternal role, especially for the youngest brother, Wataru, whom he cares for as a father figure. He is deeply devoted to his family and would give anything for his siblings. As the eldest, he carries the responsibility of looking after his younger brothers and often acts as a stabilizing presence within the household. His role in the story is that of the caring elder brother and a main male protagonist, whose motivations center on protecting and supporting his family while navigating the complex dynamics that arise with the arrival of their new stepsister. Masaomi is well-versed in medicine and uses his knowledge to care for those around him.