Masaomi Asahina is the eldest of the thirteen Asahina brothers and works as a pediatrician, a profession that suits his gentle and caring nature. He is known for being very kind and exceptionally good with children. His personality is laid-back; he prefers to spend his free time relaxing at home rather than engaging in high-energy activities. As the oldest brother, he has shouldered a significant sense of responsibility from an early age, looking after his younger siblings and, after the death of their father, acting as a father figure in particular to his brother Wataru. He cares deeply about his family and would give anything for them. In the story, Masaomi serves as a stabilizing presence among the brothers and often tries to mediate or offer calm guidance when tensions arise. His primary motivation is the well-being and happiness of his family, which includes his developing relationship with his new step-sister Ema Hinata, whose arrival sets the events of the series in motion. His most notable ability is his extensive medical knowledge as a doctor, and he is well-versed in medicine. His relationships are centered on his bond with his brothers—especially the younger ones he helped raise—and his protective, though sometimes reserved, interactions with Ema. His character development is subtle, reflecting his ongoing effort to balance his professional career with his duties as the eldest brother and his growing feelings toward Ema.