Honestly, I have no idea. I read the manga pretty fast so I don't remember everything, but if I recall, isn't the only info we have about Polaris that they're a butler ?
Given we know the stars are bizarre dolls, and we've seen at least two to be people known to the main cast (r!Ciel and Doll, I don't recall if we saw the other stars…), maybe they're someone we know… I will admit my first thought when seeing the butler one was Agni but it's not logical since he wasn't dead yet and later Soma had him cremated.
But of the top of my head, outside of Sebastian and Soma, he's the only "butler" character I could think of. So maybe it's someone we haven't met at all yet ?

Sorry, I'm thinking outloud but I'm not very good at making predictions on events to happen ^^' I'm better (I think) at analyzing what we already have.