Kaoruko Usami is the maternal cousin of the renowned novelist Akihiko Usami. She is a young woman whose age appears differently across adaptations, presented as 19 in the original manga and 20 in the anime. Physically, Kaoruko has straight, dark hair that falls past her shoulders with a distinct square-cut fringe, and dark grey eyes. She is notably shorter than Misaki Takahashi, the main protagonist. Her wardrobe is markedly traditional, as she is almost always dressed in a formal Japanese kimono, often complemented by an ornament in her hair.

At first glance, Kaoruko projects the image of a snobbish and demanding rich girl. When she first arrives at her cousin's home, she promptly orders Misaki around as if he were a servant, showing little respect for his position in the household. Her manner is consistently blunt and hardheaded, and she is not afraid to speak her mind in a direct and sometimes rude fashion. She is also intensely forward about getting what she wants, employing tactics ranging from bribery with rare collector's items like a robotic bear to simply yelling at people until they comply. In these respects, she shares a great deal of her temperament with her cousin Akihiko, displaying a similarly relentless and self-serving attitude.

However, this brazen exterior conceals a more complex and vulnerable inner world. Her primary motivation throughout her story is a desperate desire for autonomy and acceptance. She initially seeks out Akihiko because she has been tricked by her father into returning from England under the false pretense of a family emergency, only to discover she is expected to marry her own relative, Haruhiko Usami, Akihiko's older half-brother. To escape this unwanted arranged marriage, which she considers outdated, she insists that Akihiko marry her instead, not out of genuine love, but to secure his protection. Her actions are driven by a deep-seated frustration over how others constantly make decisions for her, and a yearning for her family to care about her true feelings.

In the context of the story, Kaoruko serves as a catalyst for tension and personal revelation. Her intrusion into Akihiko's household forces Misaki to confront his own insecurities about his place in Akihiko's life. She is remarkably unfazed upon walking in on an intimate moment between the two men, simply telling them to finish up so they can continue their conversation. Her bluntness extends to directly questioning Misaki about his sexuality, though she quickly declares she does not care, as her only interest is in Akihiko's protection. This encounter, however, leads to a surprising bond with Misaki. In a moment of vulnerability, she reveals her true, heartfelt dream is to become a pastry chef, a goal she is deeply defensive about, fearing ridicule. It is Misaki's unflinching support for this dream that touches her, leading her to ask for his help in recreating a castle made of cake.

Kaoruko's key relationships are central to her character development. Her dynamic with Akihiko is one of mutual understanding based on their shared stubborn nature, though she uses him more as a shield than a romantic interest. Her relationship with Misaki evolves significantly from one of condescending employer-servant to a genuine friendship built on mutual respect and support. Despite her initial rudeness, she inadvertently admits to liking him, confirming a family trait of having similar tastes in men. Notably, her most significant relationship is with another cousin, Mizuki Shiiba. Their relationship is initially characterized by bitter sniping and jealousy, particularly from Mizuki. However, it is revealed that beneath their constant bickering, Mizuki is in love with her, and she reciprocates his feelings. In this relationship, she puts on a brave front to hide her weaknesses, a side of her that Mizuki finds endearing. Her philosophy regarding love, that she simply wants the person she loves to be happy, further showcases her hidden depth.

Kaoruko exhibits notable growth over the course of the series. Her development is subtle, moving from a purely self-serving individual trying to escape a marriage to someone who offers genuine support to others. She comes to accept and support Misaki's relationship with Akihiko, even going so far as to use a private jet to deliver a Christmas cake for him. This shows she has moved past her own troubles and can invest emotionally in the happiness of those around her. Ultimately, her story arc sees her find a more authentic path, pursuing her dream of becoming a pastry chef, a goal she mentions involving a move to Paris. Her abilities, therefore, lie not in combat or special skills, but in her unwavering determination to achieve her goals, her blunt honesty that cuts through pretense, and her concealed culinary aspirations.