Haruko is the little sister of Matagu Shidou, who is always watching her and doting on her. She is also the friend and classmate of Karen Onodera and Miina Miyafuji. She is a bit naive, and it always happy and energetic.

Spoiler

After Miina and Karen work out who the sister is, Miina runs away and spends some time living at Haruko's home.