Created by Fujio Akatsuka's assistant Kenichiro Takai for the *Osomatsu-kun* series, Dayōn was initially introduced as Da Yon or Dayōn no ojisan before adopting his permanent name. This name stems from his distinctive verbal tic of ending sentences with "dayōn" or "yon." He possesses a slender body of average height and an abnormally wide mouth and cheeks. His head shape tapers narrowly at the top and widens dramatically toward the bottom, emphasizing his exaggerated mouth. He typically sports facial stubble and a long, curly mustache, wearing a khaki-colored suit with either geta sandals or modern leather dress shoes depending on the adaptation.

Dayōn is generally portrayed as an easy-going yet enigmatic figure, officially described as "unclear" and not easily understood. His wide mouth facilitates versatile gags, such as consuming large quantities of food (like 50 apples), attempting to swallow people or objects, or forming snowballs by spitting compacted snow. An early manga appearance showed an affinity for haiku composition. He lacks a fixed occupation or consistent relationships, instead filling minor roles per story needs, ranging from policeman, shopkeeper, pilot, ship captain, or truck driver. He has occasionally appeared with brothers separated during WWII or a wife sharing his facial features. His dynamic with characters like Iyami fluctuates between neutral, friendly, or antagonistic based on narrative requirements.

Across all official media—including the original 1962–1969 manga, 1966 and 1988 anime adaptations, films like *Osomatsu-kun: Suika no Hoshi Kara Konnichiwa zansu!* (1989), and the 2015 spinoff *Osomatsu-san*—Dayōn maintains his core characteristics without significant backstory or development. He consistently functions as a peripheral bit character for comedic support. In *Osomatsu-san*, he adopts a more cryptic "phantom" persona but remains a minor figure appearing randomly to perform odd actions or speak his trademark phrases. His design was reportedly inspired by Kiyoichi Tsunoda pressing his face against glass. No canonical evolution in his role, relationships, or personal history occurs across any seasons, films, or spinoffs.