Haruko Shidou is a supporting character from the anime series. She is the younger sister of Matagu Shidou, who is known for being very protective and doting on her. In contrast to her brother's intense demeanor, Haruko has a very bubbly, hyperactive, and energetic personality. She is often described as being somewhat naive and immature, appearing oblivious to mature or adult matters, even while acknowledging that her brother can be a pervert. Physically, she is depicted as a teenage girl.

Within the story, Haruko serves as a friend and classmate to the main characters, Karen Onodera and Miina Miyafuji, and is part of their social circle at school. Her role is largely that of a cheerful and supportive presence amidst the series' central romantic confusion and identity mystery. Her home becomes a significant location in the plot when, after the central mystery about the true sibling relationship is partially resolved, the character Miina runs away from her living situation and goes to stay at Haruko's house.

Haruko's key relationships are primarily defined by family and friendship. Her relationship with her brother Matagu is a notable contrast, as he is constantly watching over her while she maintains a more carefree and innocent attitude toward his obsessive tendencies. She also shares a close friendship with Karen and Miina, welcoming them and providing a degree of normalcy and warmth. In terms of development, her character remains consistently supportive throughout the series, with her most notable action being the aid she offers to Miina in a time of personal distress. Haruko possesses no notable supernatural or special abilities; her defining traits are her relentless cheerfulness and her function as a loyal friend and the innocent younger sister to a deeply concerned older brother.