The Sandman, also called Steven or Steve is the king of the Realm of Slumber.
In his first appearance he attempts to escape to the waking world, to rule it. Steven ended up falling on a small abandoned island with an old man. At some unspecified time and by some unknown means, Steven managed to leave the dream world as he intended in his first appearance but his new life wasn't so good as he expected (he ended up working as a dishwasher) so he tried to open a portal to return. 
Because it would harm innocent people to open that portal, Juniper Lee kept stopping him. When Steven explained he just wanted to return to his world, Monroe mentioned an already existing portal. When about to cross it, he remembered he didn't leave a notice at the fast food place he worked at so he asked Monroe to cover for his shift.
Steven is used to hiring help and has previously employed humans who were asleep (imprisoned in sleep by him to work on his portal) and Pirados Demons.