Guppy is a member of the Swimmers, a family of bandits operating in Sand Land. He is one of the three sons of the group's leader, who is known only as Papa. Guppy is notable for being the physically largest member of his family, with a heavy-set or obese build. His appearance is distinctive, as he wears a yellow swimming cap and goggles, and typically carries a large blue container on his back. This outlandish, swim-themed attire is a family trait, though ironically, none of the Swimmers had ever encountered enough water to actually swim before the events of the story.

In terms of personality, Guppy is often characterized by his passivity and apparent boredom. Unlike his more active brothers, he frequently appears lazy and requires coaxing or direct orders from his father to engage in the family's activities. Despite this lack of enthusiasm for their criminal work, he remains a loyal follower of his father, Papa, and is a willing participant in the family's schemes. Like the rest of the Swimmers, he is a minor antagonist, driven primarily by self-preservation and the opportunity to collect bounties.

Guppy's primary role in the story is as an obstacle for the protagonists. He contributes to the Swimmers' efforts to capture the main characters for a reward, leveraging his unique combat abilities. His notable ability lies in his choice of weaponry; he wields a very large, powerful homemade gun that is capable of long-range sniping, delivering a tremendous force against any target. This makes him the family's heavy hitter and long-range specialist, contrasting with his brother Pike's superhuman eyesight and Shark's incredible speed.

The character undergoes a significant development from an antagonist to a supporting figure. After a decisive confrontation where the demon prince Beelzebub easily defeats Guppy and his entire family, the Swimmers abandon their bandit lifestyle. They are later inspired by the protagonists' quest to find a hidden water source. In a key moment, Guppy uses his strength to help destroy a dam, releasing a flood of water to end the drought plaguing the land. He and his family then become helpful allies. They are later seen finally learning to swim in the new springs and assist the main characters against subsequent threats. His motivations shift from simple greed to a more wholesome desire, shared by his father and brothers, to experience water and live a better life. His name, Guppy, is ironically chosen, as a guppy is a small and common type of fish, which contrasts sharply with his own large size.