Despite the name, a Demon is not an evil spirit/being/entity/etc. beyond the realm of the living. Demons in the Kirby series are actually a species, related to the Fairies, that once resided on the Iron Star. The Demons first appeared in Kirby: Onslaught as minions to Dark Matter, fueling the evil force's return. There are now very few Demons in existence, having had their homeworld destroyed.
The ancestors of Demons were originally Fairies who sided with Mind Matter when he attacked the Ripple Star. Being on the losing side, they traveled with Mind Matter to the Iron Star, which became their new home. The estranged Fairies were gradually mutated into the Demon race.
The Demonic Onslaught
After six centuries of isolation the Demons, under the influence of Dark Matter and fueled by their advanced technology, let out a massivve assault on various planets. The first one to fall to them was the Ripple Star, while the attacks on the Pop Star and Sand Star were immediately repelled, and their attempt to seize the Shiver Star became a stalemate. All of the planets were eventually freed by Kirby and his allies.
Eventually, Kirby was able to reach the Iron Star, which housed Zero, who was defeated. The immenent explosion completely erased the Iron Star from the map, thus killing almost every Demon in existence.
Demons appear similar to Fairies, but are slightly taller and more slender, have small horns, fangs, red eyes, and bat-like wings rather than the insect-like wings the Fairies have.