Haruna is a central character in Onsen Yōsei Hakone-chan, primarily defined by her role as the object of the protagonist Tōya's long-standing affections. She is a young woman whose family operates the Shunjusou, a traditional Japanese ryokan or hot spring inn located in the Yumoto district of Hakone, near Tōya's own home and family business. Physically, she is depicted with long black hair that reaches her hips and brown eyes, and she is typically seen wearing her school uniform. Importantly, she is established to be one year older than Tōya, marking her as his senior and childhood friend.

In terms of personality, Haruna initially presents a calm and gentle demeanor. She is polite and kind, as demonstrated when she thanks the newly awakened hot spring spirit, Hakone, for protecting the local springs, which delights the fairy. However, this gentle exterior conceals a much more formidable and strict side to her character. When provoked, particularly by situations she deems inappropriate, Haruna reveals a personality so severe and intimidating that those around her become genuinely fearful. This strict aspect of her character is consistently triggered by her jealousy; she becomes notably stern whenever she suspects Tōya is becoming too close or flirtatious with other girls, including the other hot spring spirits like Miya.

Her primary role in the story is as the catalyst for the main plot. Tōya’s wish to get closer to Haruna is what inadvertently awakens the weakened spirit Hakone from her long slumber. This wish sets the entire series into motion, with Hakone agreeing to help Tōya win Haruna’s heart in exchange for offerings of steamed buns. Haruna is the goal of Tōya's romantic endeavors, and much of the early comedy and interaction involves Hakone's clumsy, magic-assisted attempts to bring the two together, such as using a magical puzzle box to temporarily make Haruna fall for him. Despite being the target of his crush, Haruna seems largely unaware of the supernatural machinations happening around her, reacting more directly to Tōya's own actions and interactions with others.

Her key relationships are with Tōya, the hot spring spirit Hakone, and her younger sister, Aki. As Tōya's childhood friend and neighbor, she shares a long history with him, though their relationship is complicated by her hidden jealous streak. She acts as an older figure to the child-like Hakone, often treating her with a mix of kindness and pragmatic discipline, such as insisting the weakened fairy soak in the hot spring to recover rather than leaving immediately. Haruna also has a younger sister named Aki, who often accompanies the main group on their various errands and adventures.

Throughout the series, Haruna shows a subtle development in that her calm composure is increasingly tested by the chaotic situations caused by Hakone and the arrival of other hot spring spirits. Her primary development is the revelation and reinforcement of her strict, jealous nature, which provides a running gag and a source of tension, as Tōya must navigate her wrath whenever he is perceived to be straying. While she does not possess any supernatural abilities like the hot spring spirits, her notable "ability" is her powerful, almost fearsome authority when she is angered, which commands respect and fear from her peers and even the superhuman characters.