Not about the teeth, he surely isn't a demon. It's obvious he is one of the bizarre dolls, probably one of the lord- my theory is he is Sebastian's former contractor, our shitsuji killed him, and undertaker revived him (my theory could be wrong as well, after all, it's hard to predict Toboso-sensei's next move ٩(๑❛ᴗ❛๑)۶)
I don't think so because why will he be looking so desperately for the demon who killed him? Maybe he was one of the victims seduced by Sebastian on one of his missions(〜￣△￣)〜
That could be true...when Our Ciel asks Sebastian to question people he goes by himself so who knows who Sebastian encountered during those times. He also did his own investigation on Undertaker and took his lockets to be looked at. I don't think the guy is a demon though.
He probably did seduce people because he's a demon, I don't think Sebastian behaved at all. He can't make any other soul deals but he can still seduce people to prey after them for fun. Sebastian doesn't need to sleep so what did he do to past the time? Hmm..
His teeth looks like a demon's, so not a human for sure.