Topo Gigio, an anthropomorphic mouse originating from Italy in 1959, stands 20 centimeters tall with a stout, pinkish body and a notably large head. Distinctive traits include oversized round ears, wide blue eyes accentuated by lashes, a small snout lacking a visible nose, subtle whiskers, and a perpetually smiling mouth featuring a single protruding tooth. His blonde "pagetto" hairstyle complements a wardrobe of pastel-striped sweaters, often in white and light blue, paired with trousers.

In the 1988 anime adaptation, he is reimagined as a mouse astronaut from the year 2,388,400, stranded on modern Earth after a spacecraft malfunction. Befriending a local mouse colony, he is adopted by a human girl named Gina while clashing with Megalo, a vengeful cat antagonist.

Portrayed as innately friendly and humorous, Topo embodies childlike innocence with a touch of whimsical romanticism. His mischievous streak emerges in the anime, where he taunts Megalo for amusement, yet his core remains kind-hearted, forging friendships through helpfulness. A fervent lover of Gruyère cheese, he humorously fears cats, dubbing songs referencing them as "canzonette" to soften their menace. These traits persist across iterations, blending playful antics with tender sincerity.