Gaku, a brown hare with striking green eyes, possesses notable physical strength that makes him the strongest among his peers. As the eldest sibling in a family of seven rabbit children, he shoulders significant responsibility, primarily caring for his six younger brothers—the sextuplets Tip, Top, Tap, Flip, Flop, and Flap—who adore him. This caretaker role sometimes requires him to miss activities, whether helping his parents during their illnesses or managing his brothers' needs. Their admiration drives them to imitate his nurturing behavior, even tending to a rock they believed was an egg.

Loyal and protective, Gaku fiercely defends his friends and family, showing little patience for perceived threats. While generally friendly, he maintains a certain reserve. His protective nature surfaces when confronting dangers to friends like Mallow or shielding his brothers. Gaku also harbored a hidden romantic interest in Thistle, a rabbit from Sunny Meadow Forest, initially concealed because she first corresponded disguised as a male. Their relationship blossomed into mutual affection upon meeting and ultimately led to marriage.

Throughout his narrative, Gaku matures in his responsibilities and relationships. Early challenges involve mediating sibling conflicts, such as reconciling his brothers with Clover after a broken necklace by organizing a collaborative gift-making effort. He demonstrates leadership by mobilizing the forest community during crises like fires. His journey concludes with marriage to Thistle and the birth of their son, Chard, marking his transition into familial adulthood.