TV-Series
Description
Chuubei is a clever forest mouse who encounters the lost horse Midori Makibaō searching for his mother. At this first meeting, Chuubei dubs Makibaō "Unkotarezo" and declares himself the horse's boss after they escape wild dogs together.

Initially playful and cheeky, Chuubei's attitude changes after witnessing Makibaō lose to his rival, Cascade. This defeat drives Chuubei to dedicate himself seriously to Makibaō's training, convinced the horse is extraordinary and needs unconventional racing methods.

During races, Chuubei perches on Makibaō's head, providing vital encouragement and tactical direction. He spurs Makibaō forward by biting his head, an unorthodox replacement for a whip. Makibaō's performance falters significantly without Chuubei's presence.

Chuubei's role shifts when professional jockey Kansuke Yamamoto joins. Feeling redundant, Chuubei considers leaving, stating "you don’t need two drivers in one car." Persuaded by Genjiro Obu's advice that "cars also have a passenger seat," Chuubei embraces a collaborative role with Kansuke, focusing on motivation instead of direct jockeying.

In the Satsuki Sho race, Chuubei attempts a risky technique to activate Makibaō's "Mustang Special" by pulling his forelock downward. He falls near the finish line, disrupting the race and allowing Cascade to win. The fall severely injures Chuubei's spine, ending his racing career.

Despite medical warnings that participation could be fatal, Chuubei insists on supporting Makibaō in the Japan Derby. He initially stays in Kansuke's pocket per doctor's orders but leaps onto Makibaō's head for the final stretch. His encouragement helps Makibaō achieve a tie with Cascade, but Chuubei dies shortly after seeing this result. His death profoundly affects Makibaō, causing grief and hallucinations, which the horse eventually overcomes to honor Chuubei.

In the anime adaptation, Chuubei survives the Japan Derby but retires from racing due to his injuries. He transitions to a supportive off-track role, assisting with training and strategy without further physical racing.

Chuubei is bold, fearless, and sharp-tongued. He detests losing and can be hot-headed, yet beneath this lies deep loyalty and care for his friends. Cascade acknowledges Chuubei's significance, calling him "the greatest jockey I’ve ever known."