The Redcaps are very idiosyncratic fairie beings that can be dangerous to normal people, but are only irritants to major heroes like Kevin Matchstick or Kirby Hero, although they proved fatal to the Hornblower.
 
They are often summoned to our plane in great numbers.  Matchstick faced an entire army of them in the Styx Casino.  They generally look alike, but the army at the Styx featured some sort of chieftain on a goat-like animal.  They are usually commanded by a Grackleflint or other similar being.

 
Their eponymous red caps are red from being dipped in their victim's blood. When they are "killed" (really they are just returned to their original place) they are said to leave their single tooth and their pants (although often they appear with no pants, just a coat).  They wear heavy metal boots with spikes on the bottom, which means that they make quite a bit of noise as they attack.
 

They use slings as their primary weapon, which they use to launch mystic pellets that can be very dangerous.  When Matchstick first faced the Redcaps, Mirth shielded him from their slings, saying he wasn't sure if they would hurt him.  Later on, Matchstick withstood scores if not hundreds of the pellets without even blinking.
 
They can also apparently control the weather to some degree.  At one point, one of them makes it rain to force a homeless man (actually Kevin Matchstick) to seek shelter, after which the Grackleflints try to corner him and find out if he is the Fisher King.