There are three types of enemies that operate along the lines of "nuisance, problem, unavoidable problem":
1) Minion
2) Hulk
3) Boss
Minions mostly there to show areas where player wasnt before, they are weak and can be killed in one or two hits, but in numbers, as in everywhere else, they may cause serious problems.

Hulks, on the other hand, is an enemy that resists your normal attacks, and can be defeated by a combination of skill usage, as well as having some abilities akin to the player.

Each boss is unique: Banshee, for example, is flying around haunting for any living thing, including but not limiting to the player, or Joker, who mostly doesnt directly attack player, but may or may not put some kind of traps, seal doors you passed some time ago, so that you have make a lot of noise to get out, and many other non-direct ways to harm the player.

P.S. yes i know that these two have different drawing style, joker will have same style as banshee.