Haruna Inui is Mikiko Amane and Nekota Kaname's elementary school classmate. Initially positioning herself as the leader among the girls, she displays an egoistic demeanor fueled by resentment over Mikiko's interest in Nekota, whom Haruna finds cool and laid-back. This resentment drives her to cause trouble for Mikiko, engineering attempts to ostracize her through manipulated dodge-ball matches and rumor-spreading, though these tactics consistently backfire.

Significant development follows confrontations with Mikiko and Nekota. Haruna reflects on her behavior, abandons her spiteful attitude, and integrates into Mikiko's social circle. She evolves into a genuinely caring friend who deeply values these relationships, considering Mikiko and the others her closest companions. At graduation, she emotionally expresses a heartfelt belief in their enduring friendship beyond elementary school upon receiving a farewell card.

Haruna has an unnamed older sister. Academically, she excels in Japanese Literature and Home Economics and has noted interests in sweets and shōjo manga. After elementary school, she attends a different middle school than Mikiko and Nekota.

Her relational dynamics shift profoundly: she transitions from resenting Mikiko to regarding her as a trusted confidant, crediting Mikiko for enabling her openness. Romantically, her focus moves from Nekota to Kitahara Yukke, sparked during a camping trip where Yukke becomes prominent. When Yukke confesses, Haruna skips school due to emotional turmoil. With Mikiko's intervention, the pair reconciles and begins dating.

Her background concludes with elementary school graduation, reaffirming her commitment to maintaining friendships despite separate middle schools, demonstrating her growth from self-centeredness to loyal, emotionally expressive friendship.