Polaris killed Agni, so that can't be his identity.
Madam Red wasn't a butler, so it's not her (but she could still be one of the others).
Joker was technically a butler, but he shouldn't care this much about Ciel.
Doll wasn't as tall as Polaris seems to be, and was also not a butler.
I was hoping he would be someone we knew previously, but I can only assume Polaris is a new character, one who used to be a butler for the Phantomhives, just of a rank below Tanaka. Perhaps one of the guys in chapter 133, page 19/20 or at the start of chapter 134? There was also a butler's corpse O!Ciel tripped over in that chapter; maybe that's him, for whatever reason?