Koushi-kun, called Oxnard in English adaptations, is a hamster with an insatiable appetite and a signature sunflower seed carried as a comforting talisman. His spotted, calf-like fur mirrors his Japanese name’s meaning—"calf." Initially skittish and prone to fright, he evolves into a resilient figure through shared adventures, balancing lingering insecurities with steadfast determination.

He forges a brotherly bond with Hamtaro, becoming his first friend in a new home. Though their connection endures, Koushi-kun eventually moves in with Pepper, a hamster he harbors a longstanding crush on. His tarot card readings, which inadvertently charm female peers, stir playful rivalry among male companions, adding levity to his earnest romantic pursuits.

Koushi-kun idolizes Boss, an older hamster, often mimicking his bravado with humorously clumsy results. Despite these missteps, he grows to face fears head-on, risking his safety to aid friends. His loyalty and gradual self-assurance cement his role as both a comic presence and an emotionally grounded ally.

Later narratives showcase him mentoring younger hamsters, while his dynamic with Pepper shifts toward mutual respect, though his affection persists. His storylines weave slapstick humor—overindulgence in snacks, endearing clumsiness—with poignant displays of vulnerability, underscoring a journey from self-doubt to quiet heroism.