Watayuki Amatsuka occupies the thirteenth position among nineteen sisters in the Amatsuka household, placing her as one of the middle children. She is a six-year-old first-grade elementary student. Her appearance features distinct blue hip-length curly hair, typically adorned with a hair ribbon, and vivid red eyes that enhance her youthful, innocent demeanor. Watayuki exhibits a shy yet deeply affectionate personality, particularly toward her older brother figure Yotaro. She frequently blushes when trying to interact with or approach him, addressing him as "onii-chan" with visible adoration, reflecting her desire for his attention and approval. She owns three pet ferrets, highlighting her gentle and nurturing disposition. Watayuki is often depicted alongside Fubuki, the twelfth daughter, underscoring their close bond as the youngest siblings. Their shared activities and consistent mutual presence emphasize this companionship. During the family river outing in the OVA "Baby Princess 3D Paradise 0 [Love]," Watayuki wears a bikini and participates in childhood activities. Her behavior remains consistent—displaying characteristic bashfulness, especially in group interactions involving Yotaro.