Niko's annoying traits are few and far between, but the main thing that is starting to aggravate me is he's a total whiner. I know he understands most of what I say to him because of his many responses (and he's VERY vocal), but if I tell him 'no' about something, he just follows me around with this soft, pitiful, pleading, whine.
Eventually, I'll get annoyed and scold him with a, "I say no!" and then he responds with a sharp, defiant "Mah!". To which I'll usually say something like, "Don't you yell at me," and he'll go back to the quiet, albeit frustrating, whining... and the cycle starts all over again. *snickers* He's such a back-talker.
Oh, and we can't leave out how he walks /right/ in front of me and blends into the darkness in my main hallway. He likes to lead me down the hall towards the kitchen, and suddenly drop to the floor and sprawl out so that either I trip over him, or inadvertently step on him. And of course, it's /my/ fault. :P