Gonbei is the pet cat of the protagonist, Seiko Nagare. He is a black-and-white feline who accompanies Seiko on her trip from Tokyo to Nagasaki after she is suspended from school. His primary role in the story is as Seiko's constant travel companion and a source of comic relief.

Gonbei displays a notably obedient and adaptable temperament for a cat. He is often carried in Seiko's backpack, content to stay there for long periods during their journey. His behavior is sometimes described as being more reminiscent of a loyal dog than a typical independent cat, as he follows Seiko and the other characters around without fuss. He is also known to perform specific, human-like gestures.

Gonbei has a very specific and memorable ability. He is capable of performing the traditional Japanese gesture known as akanbe, where one pulls down a lower eyelid and sticks out their tongue as a sign of defiance or rudeness. Gonbei notably directs this gesture at the character Chuuta Misora at one point in the OVA. His role in the plot is largely observational. As Seiko's pet, he is present for the key events of the murder mystery she becomes entangled in, but his function is to provide lighthearted moments rather than to actively assist in solving the crime. His main relationship is with his owner, Seiko. He does not undergo any significant personal development or change throughout the story.