Dayon is a recurring character created by Fujio Akatsuka and Kenichiro Takai, first introduced as a supporting cast member in the manga and later adaptations. The character was developed at Akatsuka's request for new cast members, with the name originating from a verbal tic used to end sentences.

Known for extreme versatility, Dayon frequently appears in minor roles such as a policeman or shopkeeper, adapting to the needs of each story. In later adaptations, the character often serves as a cryptic, phantom-like figure who smiles and engages in strange behaviors.

Physically, Dayon possesses a large, exaggerated mouth that stretches his face into unrealistic proportions. He is consistently depicted smiling, with stubble growth across his face and a long, curly mustache. His hair is neatly slicked and parted, and he typically wears a khaki-colored suit. Footwear varies between adaptations, ranging from geta sandals to modern leather dress shoes.

His personality is simple and easy-going, though it shifts according to his assigned role. Official descriptions label him as an unclear man whose identity is difficult to grasp. His large mouth is central to gluttony-based gags, enabling him to consume food, people, and garbage, and is sometimes used for practical tasks like forming snowballs. An early appearance revealed a fondness for haiku.

Dayon's speech is defined by ending sentences with "dayōn" or simply "yon."

Relationships with other characters are flexible, shifting from neutral to friendly or antagonistic depending on the story, including interactions with figures like Iyami. One specific story reveals him as one of three brothers separated during WWII and later reunited, each with different professions such as pilot, ship captain, and truck driver. At times, he is depicted as married to a woman who shares identical facial features, minus the mustache.

The character appears across various Fujio Akatsuka works, including Osomatsu-kun, The Genius Bakabon, and Extraordinary Ataro, in roles ranging from minor support to occasional protagonist. He is part of Akatsuka's Star System, appearing in multiple series under different contexts.

In animated adaptations, Dayon features in various Osomatsu-kun series, Furious Ataro, Heisei Genius Bakabon, Rerere's Genius Bakabon, and the Osomatsu-san spin-off, with his portrayal remaining consistent to the original manga.

Dayon's face was modeled after Kiyoichi Tsunoda, the elder brother of Jiro Tsunoda, inspired by an incident where he pressed his face against glass as a joke.