Demons are the inhabitants of Demon World and its capital city Demonopolis. In the battle of "good and evil", demons are considered to be evil and want nothing more than to either kill humans or use them as slaves. Grimomen was once their leader and took them to Monster World on a "sales trip" to round up the inhabitants and drag them back to their world. Physical and brutish, while being fairly skilled with a sword, the large number of demons prevented Shion from rescuing his friends. The demons moved to Fisherton and rounded up everyone there, before taking to The Great Deep, home of the dimension portal back to their home.
Other demons were seen in Demonopolis to enjoy normal jobs that wouldn't be out of place in the real world. Rough Schwartz and Egger were Demon World's immigrant control and would impose very harsh measures on visitors who didn't have the right paperwork. Drum is the landlord of The Shame Inn and is a very loyal supporter of Grimomen. When he believes Shion to work for Grimomen, Drum happily gives away all the information on how to find the winged demon.
Grimomen's plan was to use magic to turn human warriors into demon ones that will happily kill on his command. Furthermore, he wished for Shion to become a demon and be his right-hand "man". When Shion destroyed his wings, Grimomen decided to kill Shion instead, but Shion got there first and ended his rule of Demonopolis. With Grimomen gone, it is unknown who filled the void and who exactly Drum looked upto from then on.
Away from Wonder Boy, a team of footballers in the Mutant League series are called the Misfit Demons.