Kaoru Yamaba is a student at the high school attended by the main characters and serves as the charismatic and feared leader of the famous all-female gal group known as the G4. Her nickname, Yamanba, is derived from her distinct and heavy ganguro-style makeup, which is a key part of her intimidating public persona. However, this striking appearance serves a dual purpose: it is both a fashion statement and a deliberate method to conceal bruises on her face, which are the result of the domestic abuse she and her mother suffer at the hands of her father, a former boxer.

On the surface, Kaoru has a rough, bold, and outspoken personality. She has a short temper and is known for her harsh, sharp tongue, which makes her a formidable presence in the school's social hierarchy. So powerful is her influence and reputation that even upperclassmen like Ken Shinozuka are said to be intimidated by her. Despite this fearsome exterior, which leads many to label her a bully, Kaoru possesses a strong and unwavering sense of justice; she despises cruelty and never targets those who are weaker than herself. This contrast extends to a deep, hidden fondness for cute things, especially babies and small animals, which can instantly soften her demeanor. Her bedroom is filled with stuffed animals, revealing a gentle and vulnerable side she rarely shows to the world.

Kaoru's motivations and worldview are heavily shaped by her traumatic home life. Her father's violence has instilled in her a deep-seated distrust and dislike of men, making her wary and often aggressive in her interactions with them. Her leadership of the G4 group provides her with a sense of belonging, control, and authority that she lacks in her family life. Her role in the story is significantly altered by her encounter with Rihito Tatsuno. Unlike most people, Rihito treats her with kindness and without a trace of fear or prejudice, an experience so novel that it causes her to quickly develop a romantic crush on him. This sets up a classic tsundere dynamic, as her harsh way of speaking clashes with her true feelings, leading her to act bluntly and aggressively even as she tries to get closer to him.

Her relationships with other key characters are initially defined by misunderstandings. Ranmaru Mori, the 450-year-old vampire, mistakenly believes that Kaoru is bullying Rihito. This misunderstanding brings him to confront her, but it ultimately leads to a turning point where Ranmaru and Rihito become aware of her violent home situation. During a crucial moment, they help protect Kaoru and her mother from her father, an act that deepens her connection to them. In a later scheme, Ranmaru asks Kaoru to induct Rihito into the G4 group, hoping that the gyaru lifestyle will keep him away from Aoi Shinozuka, the girl he likes. This request serves to integrate Kaoru more closely into the main group and further complicates the romantic misunderstandings, as Kaoru mistakenly believes Ranmaru is interested in her. Through these interactions, Kaoru begins a gradual development, slowly breaking out of her self-imposed isolation and learning to trust others, allowing her inherent loyalty and caring nature to shine through as she becomes a more integrated part of the social circle. Her notable abilities lie not in physical strength but in her commanding presence and leadership skills; she has a natural charisma that allows her to control the atmosphere of the school and rally her group, a talent that even Ranmaru finds useful.