Hibiki Haruna is the second daughter of the Haruna family and the older sister of the protagonist, Yuu Haruna. She is a university student living in Tokyo, balancing her studies with helping to care for her younger siblings. She has a calm and reliable personality, often maintaining her composure even in tense situations. Although she typically carries herself in a reserved and serious manner, aided by her glasses, she is deeply protective of her family and demonstrates a strong sense of responsibility when the need arises.

In the story, Hibiki acts as a supportive figure for her brother Yuu. Her protective nature is clearly shown during a school festival incident where she steps in to prevent Yuu from being assaulted. This event leads to an unexpected romantic relationship with a man named Dôbayashi, who later becomes a dedicated fan of Yuu's band, Blue Wells. Hibiki’s influence extends further when she learns that Yuu is being pressured by an individual named Ikoma. She encourages Dôbayashi to expose Ikoma’s actions on social media, which generates a major scandal that helps resolve the situation. Through these actions, Hibiki proves to be a dependable older sister who actively works to protect her brother’s well-being from behind the scenes.