Hakone Oumi is Sento Oumi's younger sister. After their mother's death and with their father often away for work, she lives under her older brother Sento's guardianship. Hakone eagerly seeks to contribute to the household through chores and helping Sento. However, Sento consistently blocks her attempts, believing preventing domestic work fulfills his protective duty as her older brother. This restriction causes Hakone deep frustration and unhappiness, making her feel undervalued and unable to express her care through practical support. Her resulting chronic depression and negative emotional state mark her as a prime target for the Charger Girls (Jūden-chan), who detect and alleviate such conditions.

Despite her inner turmoil, Hakone rarely reveals her true feelings openly. She typically masks her sadness and sense of uselessness behind a facade of happiness and a carefree demeanor, leading others to view her primarily as cheerful and untroubled. This concealment complicates her relationships and sustains her vulnerability to energy charging interventions. Her deep care for Sento remains central, intensifying her distress when barred from supporting him. Hakone begins the series as a junior high school student, later entering her first year of high school. Her childhood friend, Iono Tomonaga, frequently interacts with her and the Oumi household. Hakone's inability to assist at home and Sento's dismissive attitude towards her efforts form a recurring source of her emotional struggles.