It was unusually well-organized for a webcomic, having been entirely written and laid out before its release, with a well-founded story, consistent art, and daily updates.

The comic was received favourably by critics, who praised the clever story; its puzzles and plot twists; the artwork, which balanced the often grim subject matter; and the surprising humor.

The OSS set a trap, but both parties are surprised to find that Jimmy's 10-year-old daughter, Sweet Pea, is a demon possessing the brain-damaged drunk driver.

Hunter plans to harvest Sweet Pea's blood over several months to make a demon army and replace world leaders.

Hunter wants Jimmy to witness his victory, but Sweet Pea has taken over the lab – their actual goal – and exsanguinated one hundred duplicates to demonize every human on Earth.

[2] Shiga stated that the biggest influences on Demon were the Quantum Leap television series and the films Groundhog Day and Memoirs of an Invisible Man.

[2] Shiga was also inspired by MacGyver, whose title character used intelligence to overcome obstacles with minimal resources,[1] and by Death Note, in which two brilliant minds compete for advantage.

Having the story entirely mapped out made Demon unusually organized for a webcomic, allowing him to publish seven pages per week[2] and to also display a completion bar indicating the comic's progress toward its conclusion.

[8] In the science-fiction magazine Locus, Shiga wrote that the series was primarily "a story of a man obsessed with the application of logic and science to a seemingly inexplicable supernatural phenomenon."

[2] Writing about the initial instalments of the webcomic in 2014, Rob Clough of The Comics Journal noted how Shiga's background in pure mathematics influenced the presentation of his stories as "problems waiting to be solved".

He described the webcomic as "a rapidly escalating supernatural action series" which takes as its central theme the implicit nihilism found in Shiga's previous works.

Clough praised the "clockwork intricacy" of the plot and the simple lines of the art, with red shades as the only colouring, complementing the "surprisingly visceral and fluid" action scenes.

He noted that Shiga's writing normalized the carefree violence for which Demon is known, and personally found a great deal of humour in material that might shock other readers.

[22] Cabeal felt that the gag humour had lost its impact by the end of the series, but that the story remained compelling and that Demon was unlike anything else he had ever read.

[23] In a review for the New York Journal of Books, Jake Bible called Demon "hilarious, sick, violent, disturbing [and] brilliant".

He found that Shiga's simple artistic style, devoid of unnecessary details, complemented the "absurdity and humor" of the story.

[25] The publication's Sean Edgar wrote that the series stood apart from "murder buffet" titles by its "equally brutal math and nihilism" and "unapologetic intelligence".

[1] Ziah Grace of ComicsAlliance wrote that Demon is unique and unpredictable, with long and intricate story arcs balanced with artwork that allows it to retain its humor.

[3] Chris Mautner of The Smart Set wrote that the series brings together elements of mystery, twisted dark humor, and extreme action honed in Shiga's earlier works.

He likened the series to the manga Death Note for its extreme anti-hero, stubborn antagonist, deadly puzzles, and game-changing plot twists.

Mautner felt that the series would not have worked so well without its humor and cartoonish art style, which made its darker aspects more palatable.

[27] A review in Publishers Weekly described the writing as "tight and tense", juxtaposed with Shiga's spartan "clip art" style.

[28] Graeme McMillan of The Hollywood Reporter noted that the series remained fresh due to sweeping plot twists that cause sudden reversals for the characters.

[29] Booklist called the series "smart, bizarre, and irreverent", with a high-stakes plot in which the body count was matched by the crude humor.

The reviewers felt that the simplicity of the art, with characters drawn to the point of abstraction, made the panels instantly understandable, speeding the pace of reading, and also added to the explosive moments of violence, in which gore is shown in depth and color contrasting with the paper-doll bodies.