Koharu Seto is the young daughter of Hiroko Seto, a former colleague and friend of Kousei Arima’s mother. She is a small child with fair skin, black eyes that lack visible pupils, and caramel blonde hair usually styled into pigtails. Her personality is generally bright and cheerful; she adores her mother and enjoys being around her. Initial encounters with strangers make her shy and hesitant, but given time she opens up and grows comfortable with them, eventually regarding them as part of her extended family.

She appears in the series primarily through her mother’s connection to Kousei. Her presence serves as a gentle, grounding element, often shown observing or quietly participating in moments that involve the main characters. Her relationship with Hiroko is affectionate and central to her world; she looks up to her mother with clear admiration. Over time she also forms a friendship with Nagi Aiza, and once draws a picture of Nagi playing piano with Kousei, an act that reflects her growing sense of belonging among them.

As a young child, Koharu does not have any special musical or performance abilities. She is not a musician herself and does not play an active role in the series’ musical competitions. Her significance lies in the warmth and normalcy she brings to the story, representing a simple, unconditional affection that contrasts with the more intense emotional arcs of the older characters. Her development is subtle, shown through her increasing comfort with new people and her willingness to include them in her own understanding of family.