Dayōn emerged as a supporting character created by Fujio Akatsuka and Kenichiro Takai for *Osomatsu-kun*, developed specifically at Akatsuka's request to broaden the series cast. His name stems directly from his signature verbal tic, "dayōn" or simply "yon," frequently ending his sentences.

He boasts an abnormally wide mouth that stretches his face into exaggerated proportions. Typically maintaining a smile on his slender frame, he wears a khaki-colored suit with neatly slicked, parted hair, stubble, and a long, curly mustache. His footwear varies: original manga, Studio Zero anime, and pachinko versions show him in geta sandals, while the 1988 Pierrot anime and *Osomatsu-san* depict modern leather dress shoes.

Officially described as an "unclear man whose identity cannot be easily understood," Dayōn exhibits a generally easy-going but versatile and enigmatic personality that shifts with his role. His oversized mouth facilitates gluttony-based gags, enabling him to swallow objects like food, garbage, or even people, or form snowballs for snowmen. An early *Osomatsu-kun* appearance reveals an affinity for haiku composition.

In *Osomatsu-kun*, he functions as a multipurpose background character, assuming minor roles like policeman or shopkeeper as needed, rarely taking lead parts. His portrayal evolves in the *Osomatsu-san* spin-off into a cryptic "phantom" figure performing unexplained actions while smiling. His dynamic with Iyami fluctuates between neutral, friendly, or antagonistic per story. A specific narrative, "The Sextuplets vs. The Great Gang of Japan," positions him as one of three brothers separated during WWII, reuniting as adults with distinct professions—pilot, ship captain, and truck driver. He is occasionally depicted with a wife who shares his facial structure but lacks his mustache.

His background maintains his core traits as a malleable, periphery figure throughout the franchise.