Gaku is a character who works at a cat cafe, employed there alongside his sister, Kaoru, and a colleague named Kunagi. His typical role involves serving food and drinks to the cafe's customers. He is recognizable by his purple eyes and a short beard, and he usually wears a white shirt with a yellow cat-themed apron over dark blue jeans.

In terms of personality, Gaku presents a charismatic and friendly demeanor to the public. He is known to offer advice related to cat ownership, often stating that taking care of cats is not as easy as one might assume, a sentiment he sometimes shares alongside Kunagi and Kaoru. However, his personality has a sharp edge; he becomes extremely angry if someone violates the cafe's rules. This is demonstrated when he knocks out a customer named Tanishi for wearing cologne, angrily declaring that such scents are not allowed near the cats.

Gaku’s primary motivation is the safety and well-being of those around him, particularly his sister. When the Nyandemic begins, he is one of the first victims, infected by his own cat that had come into contact with the origin of the virus. Despite his dire situation, his focus remains on protecting others. While showing the physical signs of the infection, he uses his remaining humanity to try and slow down the advancing cats, giving his sister, Kunagi, and Tanishi a chance to escape. He faces his impending transformation with a sense of peace, even happily embracing what he is about to become.

His role in the story is a catalyst for the initial survival journey. His infection and sacrifice serve as the event that forces the other main characters to flee the cat cafe and confront the reality of the Nyandemic. His final plea to Kunagi to protect his sister, Kaoru, establishes a key emotional driver for the group's actions early in the narrative.

Gaku’s most significant relationship is with his sister, Kaoru. Her emotional breakdown upon seeing his condition and having to leave him behind underscores the strength of their bond. He also shares a workplace relationship with Kunagi, trusting him enough to entrust Kaoru's safety to him. Beyond these key relationships, his interactions with customers like Tanishi show his dual nature as both a friendly guide and a strict enforcer of rules.

Gaku's development is tragically brief but poignant. He transitions from a charismatic cafe employee and caring brother to a self-sacrificing victim of the pandemic. His arc completes with his heroic decision to hold back the feline horde, embracing his fate to ensure the survival of his sister and her companions. He does not survive the initial outbreak, and his transformation is a pivotal, somber moment that defines the stakes for the remaining characters.

Regarding notable abilities, Gaku does not possess any special combat or survival skills. His primary ability is his knowledge of cats, which he uses in his professional life at the cafe to advise customers. His most remarkable act is a feat of willpower, as he manages to hold onto his humanity just long enough to use his body to block the pursuing cats and facilitate the escape of the others.