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The Seven Deadly Sins manga series features a cast of characters created by Nakaba Suzuki. Set in a fictitious Britannia in a time period akin to the European Middle Ages, the story references various traditions, including Christianity (e.g., the Seven Deadly Sins, the Ten Commandments) and Arthurian legend (e.g., Meliodas, Diane, Ban, Harlequin, Gowther, Merlin and Escanor). This is frequently done in ironic or contradictory ways, such as in the emergence of the title group as the protagonists and the group of "Holy Knights" as the antagonists of the series.

In accordance with the medieval theme, many of The Seven Deadly Sins are depicted as knight, who are clad in stylized variations of medieval armor, many of whom perform magic. The five major clans in the series are Human, Giant, Fairie, Goddesses, and Demon, all of which possess powerful abilities and magical powers. Suzuki's sequel to the manga, Four Knights of the Apocalypse, continues these Christian and Arthurian references (e.g., the Four Horsemen of the Apocalypse and Percival).

In order to surprise readers, Nakaba Suzuki purposely made the appearances of some of the titular group of knights very different from their wanted posters that appear in the first chapter and had others look the same as their poster, but gave them "horrible" personalities. Although it was decided to make the protagonist of the series a "child", Suzuki struggled with designing Meliodas' profile because the character is actually an adult. He said the most difficult part was his hair; ultimately deciding on "fluffy" hair like a boy from a foreign country, which he had never done before. In the pilot chapter, the character had long, straight hair, which the author said was not as cute. In the published one-shot version, Hawk and his mother did not appear. Instead, Melodas' traveling tavern, named "Apple", rode atop a giant spider-like mechanism. Suzuki said he always made sure to show the subtly different relationships between the members of the titular group of knights. For example, he said that while Ban is Meliodas' best friend, King is only a teammate with whom he does not talk. All seven have such relationships, which the author called realistically human.

The initial design for Diane was slightly smaller than the final version and had disheveled hair instead of pigtails. Suzuki explained that his editor at the time was worried about her being too big, thus they were going to go with about two meters tall. But that is not uncommon for women in manga and the author recognized that the fantasy manga was being overcome by a concern about reality. Although that version of Diane still had a crush on Meliodas, she was more hot and cold about it. Ban's initial design was very different from the final version. He did not talk much, always wore a poker face and had a personality that was difficult to "rile up". King was initially going to be named "Jehan" and represent the sin of pride. Instead of transforming into an old fat guy, the initial design transformed into a pudgy baby. But his editor at the time pointed out this looked too much like Gawain from Rising Impact. Additionally, because having a kid turn into an old guy is "kind of cliché", the author considered having the old guy be King's main form, before deciding against it. The prototype design for Gowther was that of a warlock named "Jeema", who represented the sin of sloth and who was one of the Seven Deadly Sins whose job it was to secretly protect Meliodas. He eventually evolved into a giant clad in armor, before Suzuki felt that would be boring and changed him into the final version of a glasses-wearing youth. The author modeled Gowther's appearance on a girl he had a crush on, but always drew him as male. Merlin's prototype wore glasses and originally went by the mythical figure's Welsh name, "Myrddin".