However, it's this page that convinced me of this theory.
The look on Undertaker's face is strangely vulnerable, strangely personal. He reacts as if Polaris is himself talking, and that he's describing exactly what Undertaker DID choose to do in death (as his life as a shinigami)

Polaris is full of rage and overwhelming passion for his master—perhaps because the trauma that made Undertaker take his own life is still extremely fresh in his memory and he hasn't had decades to calm down like the shinigami version of Undertaker.

So, there are only a few things things we know about Polaris:
1. He is a reanimated corpse
2. He was a butler in his previous life
3. He has lost his master at least once before (in the Phantomhive family?)
4. He is ferociously dedicated to R!Ciel (and perhaps the Phantomhives)

Who is Polaris?
I think he could be Undertaker.

(I mentioned this theory in a server and people liked it so sharing it here 🧵)

Yes!! one of the biggest kuro-centric twt accounts at the time (of which there were very few) was an anti, and they hopped on this (dumb) meme format but with sebaciel.
it blew up and brought LOADS of ex-sebaciel shippers together and a lot of general shota-themed twts pivoted to kuro (like me)

Hi everyoneee I wrote a spiritual sequel to my Ciel SH fic... Sebastian covers his scars with bite marks 🥰 #sebaciel archiveofourown.org/works/60329554

yana did NOT need to draw ciel's waist Like That... but thank god she did 😌😌