Pyonkichi originated when Hiroshi Kaizuka accidentally crushed him during a fall, leading to his reincarnation as a sentient imprint permanently attached to Hiroshi's shirt. Hiroshi's impoverished circumstances, limiting him to one good shirt, cement this existence as a "Flat Surface Frog." Pyonkichi exhibits obstinate determination and gluttony, frequently engaging in sibling-like arguments with Hiroshi while still offering him life advice.

He demonstrates surprising physical capabilities, jumping while attached to the shirt and exerting control over Hiroshi's movements. His strength allows him to lift heavy objects using his limbs and teeth. Pyonkichi develops romantic attractions across species boundaries, showing interest in female cats, frogs, and humans. This trait leads to comedic situations, such as a misunderstanding over a kiss mark causing turmoil with a feline companion.

His personality includes moments of self-sacrifice, notably when he deliberately alienates a frog girlfriend to spare her the difficulties of his unusual life. Accepted by Hiroshi's mother as family, Pyonkichi participates in school pranks against teachers, navigates complex social dynamics within Hiroshi's friend group, and faces challenges like temporary hypnotic transformation into a dog that amplifies inappropriate behavior.

Throughout, Pyonkichi maintains persistent determination, emotional vulnerability beneath a comedic exterior, and deep loyalty to Hiroshi despite their constant bickering.