Buchi is a street cat living in Tokyo who becomes one of the first friends the protagonist Rudolf makes after being separated from his home in Gifu. As a stray, he is a calico cat known for his eccentric and energetic personality. One of Buchi's most distinctive traits is his habit of mimicking martial arts moves, often punctuating his actions with a sharp cry of Hyah. This behavior gives him a nervous and somewhat hyperactive demeanor, as he can become easily distracted, particularly when a beautiful female cat catches his attention.

In the story, Buchi serves as an initial guide to Rudolf, who is lost and unprepared for the harsh realities of city life. Upon meeting Rudolf, Buchi provides valuable information about their shared environment, including details about the formidable stray cat Gottalot, whom he reveals is known on the streets as the Junk Tiger. He also warns Rudolf of the dangers present, such as a vicious dog named Devil. Buchi offers Rudolf a place to stay and shares his knowledge of local history, including a past fight between Gottalot and a Doberman.

Despite his flighty and easily distracted nature, Buchi develops into a loyal and helpful companion to Rudolf throughout his journey. He plays a key role in a crucial moment when Rudolf accidentally boards a freezer truck, chasing after the vehicle alongside Gottalot to rescue the frozen kitten. Buchi also directly assists Rudolf in a confrontation with the film's primary canine antagonist, Devil. During this fight, Buchi creates a diversion with a feint that allows Rudolf to gain the upper hand. By the conclusion of the story, Buchi finds a romantic partner in a Scottish Fold cat named Misha, and he remains a steadfast resident of Tokyo, continuing his friendship with the now-settled Rudolf.