Dekapan is a recurring character distinguished by his extremely memorable visual design. He is a short, heavyset middle-aged man who almost always appears wearing nothing but a pair of very large, striped boxer shorts. He is bald, with only a few sparse hairs at the base of his head, and his face is marked by a thick black mustache, a single prominent bucktooth, and crow's feet wrinkles around his eyes. His name, which translates to large pants, was chosen through a reader poll in Shonen Sunday magazine. The large boxers he wears are not just a stylistic choice; they are a practical tool. Due to their extreme width, they can store a vast and bizarre array of items, from everyday tools and money to live animals like cats.

In terms of personality, Dekapan is most often portrayed as a mild-mannered, big-hearted, and eccentric older uncle type. He stands in contrast to the more gaudy and loud character of Iyami, generally presenting as a friendly and easy-going figure. He has a well-documented love for animals and is frequently seen with a pet cat or dog that he calls Boya. His way of speaking is highly distinctive, as he often punctuates his speech with a hoe hoe laugh and ends his sentences with the suffix dasu. However, his character is highly flexible. While usually a force for good, he can be cast in darker, more villainous roles, such as a cold-hearted mafia boss, depending on the needs of a particular story.

Dekapan's role in the narrative is that of a versatile supporting character. He frequently appears as an authority figure with a helpful profession, most commonly as an eccentric doctor or a brilliant scientific inventor. His inventions are often the central plot device for an episode. For example, he once created a serum that allowed a cat to speak the innermost thoughts of Ichimatsu, leading to an unexpectedly dark and revealing character study. He is often seen in the company of the bizarre character Dayon, and the two form a memorable comedic duo.

His relationships with the main cast, the Matsuno sextuplets, are primarily based on his role as an inventor or helper. He shows a particular connection to Jyushimatsu, and on one occasion, he even allowed the energetic brother to live with him. While typically portrayed as living alone with only his pets for company, the character's long history has seen his family status vary. For specific storylines, he has been depicted as the father of other characters like Totoko, Chibita, or Hatabo, and in some adaptations, he has even been shown with a wife and a toddler son named Taro. This adaptability is a core trait of the character, whose appearance and mannerisms remain consistent even as his role and relationships shift from one story to the next, a quality that has allowed him to appear across many different works beyond the Osomatsu universe.