Dark's background is a mystery, although he refers to being raised in a strict religious upbringing. When asked if he reads the Bible, he replies, "I've had every page, line and word read at me, sir!" Another, perhaps more fitting interpretation of the Bible sequence, given his dismissive nature to Charles and his array of books, would be that he is mocking Charles' belief that the Bible would have any sort of effect in either repelling or hurting him.
This would indicate that Dark is not a devil or demon in the Christian sense of the term. He died by growing old when he was on the magic carousel.
He is a sinister man who is the leader of the Autumn People and he bears tattoos all over his body, one for each person successfully tempted into joining the Dark's Pandemonium Carnival that he founded and runs.
Mr. Dark initially holds sway over the other main characters, but his power weakens when Charles uses positive emotions against him, something that he cannot comprehend or withstand.
Mr. Dark with tattoos of Jim Nightshade and Will Halloway on his palms
Mr. Dark making a deal with Halloway if he tells Mr. Dark where the boys are, he'll make him young again
"My name is Mr. Dark. I advise you to respect that."
Mr. Dark in front of Mr. Copper, Will Halloway and Jim Nightshade at the Carousel
Mr. Dark with a parade
Mr. Dark with his freaks
The Truth is you're lying.
Mr. Dark dying by getting shocked and having the carousel age him