Raponchi began as a typical frog inhabiting Shakujii Park in Nerima, Tokyo, until Hiroshi, a middle school student, stumbled over a rock and accidentally crushed him. This event caused his rebirth as an indelible imprint on Hiroshi’s shirt, morphing him into a "Flat Surface Frog" (平面ガエル, Heimen Gaeru). His existence became permanently fused to the garment—Hiroshi’s sole presentable shirt—ensuring their unavoidable, constant interaction.

His personality merges obstinacy and gluttony, mirroring Hiroshi’s traits and sparking frequent sibling-like clashes alongside mutual support. Despite his flattened form, he demonstrates unexpected physical prowess, leaping great distances and often yanking Hiroshi involuntarily with him. He wields remarkable strength, lifting heavy objects using his arms and teeth. Hiroshi’s mother acknowledges him as a second son in their household.

He consistently exhibits attraction to females across species—including humans, cats, and other frogs—which shapes multiple story arcs. His former amphibian partner, Ranosa, once tracked him down, but he refused reunion due to his bond with Hiroshi’s shirt. His name reflects his recurring displays of determination or "gutsyness" against obstacles.

Raponchi’s relationship with Hiroshi shifts from initial friction to deep camaraderie, with Raponchi providing life advice and commentary on Hiroshi’s experiences. He engages actively in neighborhood disputes, notably biting the bully Gorilla-Imo’s navel during confrontations. In the Solmac advertisement series—treated as official continuity—Raponchi persists as Hiroshi and Kyoko transition into adulthood and marriage. The 2015 live-action drama extends his presence decades later, depicting him still affixed to Hiroshi’s shirt in their thirties, sustaining his role as Hiroshi’s lifelong companion and observer.