Yotsuba was abandoned by her mother. Her country of origin is unknown. How Mr. Koiwai came to meet her is unknown, but he stated that he just started taking care of her and soon enough, became her (adoptive) father.
She is a lively and lighthearted girl. She takes a challenge and makes it into a fun game. She is always playing, and is very innocent. Yotsuba is very direct, but also kind.