Pyon-Kichi began as a common frog in Tokyo's Shakujii Park until middle school student Hiroshi Kaizuka accidentally crushed him during a fall. This event transformed him into a sentient imprint, a "Flat Surface Frog," permanently bonded to Hiroshi's sole presentable shirt. He retains his original identity and name.

His existence revolves around accompanying Hiroshi and offering commentary on their daily life. Sharing traits of stubbornness and gluttony, they engage in constant sibling-like arguments while also demonstrating mutual care and support. Pyon-Kichi manipulates Hiroshi's movements by jumping while attached to the shirt and possesses sufficient strength to lift heavy objects using his limbs and teeth. Hiroshi's mother treats him as a second son.

Pyon-Kichi exhibits attraction to females of any species and readily defends Hiroshi physically, often by biting adversaries such as the bully Gorira-Imo. His name stems from frequent displays of determination. An encounter with his former frog companion, Ranosa, highlights an emotional conflict as he rejects reunification due to his bonded state with Hiroshi.

His core characteristics and circumstances remain unchanged across sequels, films, or adaptations. The 2015 television drama depicts him continuing his role with an adult Hiroshi, maintaining their dynamic. Novelizations reaffirm his origin story and persistent encouragement of Hiroshi.