Kōshi-kun is a large hamster with an insatiable appetite, frequently seen eating or carrying sunflower seeds. After Hamtaro moves to a new home, Kōshi-kun becomes his first friend, forging a strong brother-like bond. This connection extends to their owners, who are best friends. Initially shy and somewhat cowardly, Kōshi-kun gradually gains confidence through adventures and friendships, though he retains a degree of timidity.

Physically less agile than other hamsters, he runs slower and displays clumsiness. Despite this, he demonstrates loyalty and bravery when necessary, especially to protect friends. His primary love interest is Pepper, whom he initially admires from afar. Encouraged by Hamtaro, he shouts his affection to Pepper in "Oxnard's Big Crush," confessing his feelings. Their relationship evolves with frequent arguments that resolve quickly, solidifying a deep mutual affection.

Later, Kōshi-kun chooses to remain at Dylan's farm with Pepper instead of returning home with his owner, Kana Iwata. This decision culminates in their marriage during the series finale, "Happy Ham-Ham Wedding!", where they invite all known Ham-Hams to celebrate. His devotion to Pepper remains constant.

Kōshi-kun's cow-pattern fur inspired his Japanese name, derived from "ko-ushi" (子牛), meaning "calf". In English adaptations, his name merges "ox" and "Bernard". His personality balances a love for food with unwavering loyalty, making him a dependable presence within the Ham-Ham community despite occasional fears.