Spank is a small white dog with distinctive black or brown ears. His original owner vanished at sea, prompting him to wait daily at the beach for their return. This lonely vigil ends when he encounters Aiko Morimura, a junior high student grieving the recent loss of her dog, Papi, in a car accident. Despite Aiko's initial reluctance, Spank persistently enters her life and persuades her to become his owner.

He exhibits a clumsy, energetic, and big-hearted personality. Spank frequently engages in human-like behaviors: walking on two legs, eating with utensils, wearing occasional clothing, and displaying clear emotional responses. His antics often cause comedic mishaps, derailing plans or creating chaotic situations. A significant trait is his large appetite, leading to gluttonous tendencies.

Romantic pursuits form a recurring aspect of his character. He develops an intense, mostly unrequited crush on Cat, a Siamese cat owned by Aiko's classmate Serino Mihara. This fuels attempts to impress her, sometimes through misguided efforts like challenging rivals or aspiring to become a police dog. These endeavors typically end in failure, reinforcing his role as a lovable underdog. Later, his affections shift toward Fan Fan, a female dog, culminating in a more mutual relationship.

Interactions with other animals highlight his social dynamics. Spank maintains a friendly rivalry with Torakichi, a cat who also pursues Cat's affection. Their relationship mixes competition with genuine camaraderie during shared adventures. He also forms a protective bond with Baron, a puppy resembling Aiko's deceased dog Papi, acting as a big brother figure.

His relationship with Aiko serves as the emotional core. Spank provides consistent companionship, steadying her during periods of loneliness after her father's disappearance and mother's departure to Paris. His presence aids her healing from Papi's loss, though his antics occasionally cause friction. Moments of separation, such as when Aiko considers moving to Paris or Spank briefly runs away, underscore their mutual dependence.

Throughout the series, Spank displays loyalty and protective instincts. He aids Aiko during emotional crises, including conflicts involving Rei Ikegami (his former owner's brother), and assists in reuniting fractured relationships among human characters. Despite his flaws, his persistence and warmth consistently contribute to resolving conflicts and strengthening bonds within his community.

By the story's conclusion, Spank's role in fostering reconciliation and joy among both human and animal characters remains central, solidifying his place as a cherished companion despite his imperfections.