Created by Kenichiro Takai in response to a request for new cast members, Dekapan's name originated from a *Shonen Sunday* reader poll and means "large pants." This short, heavyset, middle-aged man sports a bald head with sparse hairs at the base, crow's feet wrinkles, a thick black mustache, and a single prominent bucktooth. His most defining feature is the oversized striped boxers he wears year-round, though early stories occasionally depicted him fully clothed for narrative reasons. These boxers function as storage for diverse items like tools, money, food, and live animals, a capability Fujio Pro clarified stems purely from their extreme width. For swimming, he dons black swim shorts underneath them.

Typically a mild-mannered, friendly "uncle" figure contrasting with the flamboyant Iyami, Dekapan's role proves fluid under Akatsuka's Star System. He shifts between heroic and antagonistic positions story by story. Heroic portrayals cast him in eccentric authority roles—doctor, millionaire, scientist, or policeman—aiding others with a big-hearted, easy-going nature. Conversely, darker narratives, especially when Iyami is heroic, may cast him as a villainous figure like a mafia boss, exhibiting colder, more wicked yet comical traits. His personality includes a love for animals, often calling pets "Boya," and an eccentric tendency to equate children like Chibita or Hatabō with animals. He punctuates speech with "hoe hoe" laughter and ends sentences with "-dasu."

Relationships vary significantly across media. While official profiles like the 1988 Kodansha guide describe him as solitary with no family, stories frequently contradict this. He occasionally appears as a father or uncle figure to Chibita, Hatabō, or Totoko. Rare appearances show biological family, including a young son resembling him in "Cat's Eye News," another son in "The Great Stupid Detective Kogoro Hakuchi," and the 1988 anime featured a wife and toddler son named Tarō who wore pink-and-white striped boxers.

Dekapan appears consistently across all official "Osomatsu-kun" media—the 1966 and 1988 anime series, the theatrical film "Osomatsu-kun: Suika no Hoshi Kara Konnichiwa zansu!," and spin-offs like "Mr. Osomatsu"—maintaining his core design and traits without significant evolution or backstory development. Beyond the main series, he features in numerous other Akatsuka works ("Tensai Bakabon," "Moretsu Ataro," "Akko-chan's Got a Secret!," "The Great Stupid Detective Kogoro Hakuchi"), often in minor or cameo roles, and retains his established persona in video games like "Osomatsu-san The Game: Hachamecha Shūshoku Advice," typically as a scientist or supporting figure. His portrayal remains situational, adapting to each story's comedic or narrative needs without predefined character arcs or transformative development.