Sachi Umino, biological daughter of Yohei and Namie Umino and biological younger sister of Erika Amano, was raised believing Nagi Umino was her blood-related older brother due to a hospital mix-up. After Nagi discovers his true parentage and moves in with Erika's family, Sachi, fearing abandonment, runs away to live with them, driven by her deep attachment to Nagi that blossoms into romantic feelings upon learning they share no blood. She possesses blond hair often styled in a bob cut and light blue or purple eyes. Her typical attire is her school uniform: a white top with blue accents, a blue skirt, white knee-high socks, and black shoes, consistent across official portrayals. Initially appearing indifferent, particularly toward Nagi, Sachi masks significant emotional depth and fierce determination. As a child, witnessing Nagi win a swimming award spurred her to train relentlessly until she earned the same recognition. This tenacity extends to her romantic pursuit of Nagi, actively vying for his attention despite his initial sibling view. She exhibits jealousy, notably toward Hiro Segawa and initially toward Erika, though she eventually overcomes her initial dislike, forming a close bond with her biological sister. Key developments include an accidental kiss with Nagi during a power outage, fueling her resolve to be seen romantically. Nagi eventually agrees to acknowledge her as a woman, validating her feelings. She decides to enroll in Nagi and Erika's high school after junior high. Meeting Sousuke Amano, Nagi's biological brother, causes emotional distress; Sousuke's resemblance forces Sachi to confront the reality of her non-biological relationship with Nagi. Her cooking abilities are noted. Her character arc consistently revolves around navigating complex familial and romantic dynamics, particularly her struggle to redefine her relationship with Nagi while maintaining bonds with Erika and others.