A personal pet peeve is sidekick characters that are totally random non-human beings that seems to really have no major tie to their world for being that way, mostly seen in manga. I think it works in series that are meant to be very light hearted or at least have that aspect but otherwise I feel like I see what are very blind copy paste manga template plots replicating these sort of designs with no (ironically) rhyme or reason behind it. Characters are just strange, ugly or abstract and no one else living in this world seems to mind these very alien and impossible creatures that live among them (and most annoyingly, are never explained.) It's too much to ask of my suspension of disbelief.
I guess it's the flip side of "uniquely mega nuclear cool main protrag against sea of npcs"