These are the dudes that have your back. They may fight with you at times, but they will always fight for you. Often the comic relief sidekick to the protagonist.
Ayakashi, yokai, oni, kami, youma, akuma, mushi. You name it, these shows deal with the 'other.' While a lot of anime has supernatural elements in it, some shows are more specifically about it.
Anime that mess with your head.
Characters with that sarcastic edge- they may be jaded, world-weary, anti-social, nerds or otherwise don't quite fit in with everyone around them.