Kaoru Ijuin is a high school student and a childhood friend of the protagonist, Mitsuyoshi Tada. He is a member of the school's photography club alongside Tada. His family background is notable, as he is the heir to a generations-old, prestigious restaurant in Tokyo's Ginza district. This upbringing has made him an exceptionally skilled cook. In terms of appearance, when he is silent, he comes across as a handsome and cool individual. However, this composed facade is quickly shattered by his true personality, as he is typically very noisy, flamboyant, and unable to keep quiet for long. He has a theatrical and expressive nature, enjoys showing off, and possesses a narcissistic streak, as seen in his habit of taking photographs of himself.

Kaoru's primary motivation revolves around his deep-seated loyalty to Tada. The origin of their friendship is rooted in a childhood incident where Tada saved him from a dangerous situation, leading Kaoru to regard Tada as his one and only best friend. Partly due to this history, he is dedicated to supporting Tada, particularly because Tada carries emotional pain from losing his parents. For instance, he annually organizes a personal event he calls the Ijuin Kaoru Show, a series of activities and meals held at the Tada family coffee shop designed to bring joy to Tada and his sister Yui. This role in the story establishes him as a key source of comic relief and vibrant energy, but also as a crucial emotional anchor for Tada, encouraging him to confront his true feelings rather than run from them.

Kaoru's key relationships are centered on the Tada family. He is a constant presence at the Tada Coffee Shop, where he has become good friends with Tada's grandfather and his younger sister, Yui. His relationship with the family's pet cat, Nyanko Big, is notably antagonistic; the cat strongly dislikes him, which forces Kaoru into a self-defense mode whenever they encounter each other. Within the photography club, his outgoing personality contrasts with the more reserved nature of his peers. He also has a fearful dynamic with Alexandra, known as Alec, who once mistook his intentions and violently kicked him, leaving a lasting trauma.

Throughout the narrative, Kaoru does not undergo a drastic personality shift, but his role as a steadfast friend becomes increasingly significant. His development is shown through his actions as a reliable confidant, using his perceptiveness to understand what his friends truly need. His notable abilities are a direct result of his background. Beyond his excellent cooking skills, which allow him to prepare entire feasts, he is also an enthusiastic, if sometimes self-absorbed, photographer. His energetic and dramatic flair for performance makes him a natural entertainer who brings levity to almost any situation.