Kaoru Mitarai is the younger sister of Hiroshi Mitarai, and together they are the two children of a wealthy and affluent family that becomes intertwined with the main circle of friends in the story. She is a young girl with a childish appearance, characterized by her purple hair that is typically cut short and brown eyes. Her look is often completed with a school uniform and hair accessories such as bows, reflecting her status as a student. Her birth date is March 1, making her star sign Pisces.

In terms of personality, Kaoru is depicted as determined, possessive, and somewhat combative, especially when it comes to matters of the heart. She harbors a strong romantic affection for Takashi Ayanokouji, a brilliant and teasing student who is a friend of the protagonist Kotarou. Her infatuation is so intense that she claims she would be willing to become a demon if it meant securing his love. This determination frequently manifests as a desire to interfere with others, particularly when she perceives them as threats to her romantic goals or to her family.

Kaoru believes that her brother Hiroshi has been seduced or trapped by Misha, the cheerful angel apprentice who is the series' central character. Consequently, she actively tries to separate Hiroshi from Misha, whom she dislikes. Her behavior often results in quarrels, especially with her brother, leading to a dynamic of constant bickering between the two siblings. While her actions are often positioned as comedic obstacles for the other characters, they also serve to complicate romantic tensions and rivalries within the group, particularly between Hiroshi and Takashi. For example, she once attempted to prank Takashi to avenge her brother's honor, only to fall in love with him instead, which set the stage for her ongoing role as a rival for his attention. Despite her sometimes irritating behavior, she is shown to get along with Kotarou’s other friends, though this is complicated by her status as Hiroshi's sister.

Kaoru does not possess supernatural abilities like the angels and demons in the series, but her notable ability lies in her tenacity and her willingness to scheme and meddle in the affairs of others to get what she wants. Her role in the story is primarily as a supporting character who creates conflict and comedic tension, largely through her unrequited love for Takashi and her opposition to Misha. While much of her screen time is dedicated to these comedic rivalries, her character is part of the ensemble that receives more focused emotional development in the latter part of the series, contributing to the deeper exploration of the main characters' feelings.