Put simply, Mr. Dark is the novel's bad guy. (See "Character Roles" for more on his villain status.) Along with Mr. Cooger, he is the proprietor of the carnival and, as such, is focused on luring people to accept what the carnival offers – a ride on the carousel, for instance, as most often happens to be the case. For each person that succumbs to the carnival's charms, he has a tattoo of their image placed on his body. With so many tattoos covering his body, he is known also as "The Illustrated Man."
As far as bad guys go, Mr. Dark is a crafty manipulator. He can tell, for instance, that of the two boys, Jim is far more susceptible to the carnival's attractions than Will. So he offers Jim a ticket for a free ride on the carousel. In an effort to later identify and locate the boys, he spreads a fake story about prize winners. He is chummy with the policemen and other townspeople. His friendly surface masks a truly sinister man.
Mr. Dark's final incarnation in the novel is as a boy named Jed. As Jed, Mr. Dark attempts to lure Mr. Halloway away from Will and Jim, but is ultimately foiled by Mr. Halloway's refusal to allow Evil any power. In the face of such disbelief, Mr. Dark dies. This is an important element of his character and an important lesson in the context of the novel as a whole. His death suggests that his evil existence depends on people's fear of Evil. That is, as soon as we refuse to give fear any power, fear goes away. Sounds like a good deal to us.