Because Toboso conceals his face, Polaris is probably a character who readers would recognize.

Polaris has claimed on multiple occasions to be a butler, and said that he lost his previous master. This might lead one to assume he was a butler in Vincent Phantomhive's household. However, Polaris appears to have a different chin shape compared to any of the previous butlers.

Polaris' first physical appearance is when r!Ciel comes to the townhouse. Soma opens the door, let's r!Ciel in, and offers him food because he believes him to be o!Ciel. R!Ciel shoots Soma with a small handgun, which is o!Ciel's signature weapon. When Agni tries to protect Soma, Polaris appears and stabs him with a knife, of which he has many. In this scene we see that Polaris is tall, but not as tall as Agni.

He is also incredibly strong and fast, performing maneuvers that would be impossible for a human, and as a result, impossible for a bizarre doll created from a human, as the bizarre dolls we've seen so far seem to retain the physical limitations of their human bodies.

Sebastian even remarks that killing Agni would have required more than 5 average humans.

The rest of the information we have about Polaris is very limited. In Japanese, he refers to himself using 私 (watashi) and uses uses polite sentence endings (desu and masu).

However, Polaris' most distinguishing characteristic is his devotion to his master. When confronted with the possibility of running out of blood and losing r!Ciel, he loses control, yells, stops using polite speech in Japanese, destroys his surroundings, and falls to his knees.

If Toboso were introducing a new character, she wouldn't need to hide his face. The only existing character who fits the profile of a butler, exceedingly devoted to his master, almost as tall as Angi, wielding an absurd amount of knives, and with strength and speed way beyond those of humans, is Sebastian.

When Polaris first stabs Agni in the townhouse, he seems to appear suddenly and unnoticed by Soma and Agni. The reader might assume that he came into the room or out of hiding at this moment to defend r!Ciel. However, I believe it to be more likely that he walked in with r!Ciel, unremarked-upon because if Ciel walks in, Sebastian would be expected to follow. In fact, it would be strange if the 13-year-old came in alone, without an adult, and especially without his near-constant companion.

At this point I will mention what originally made me think of this theory: Sebastian's catchphrase. I will do my best, with my extremely limited knowledge of Japanese, to explain.

The phrase Sebastian is so fond of saying is "私は[悪魔で]執事ですから." He's literally saying "For I am a devil and a butler," but doesn't blow his cover as a demon because it sounds identical to "For I am a butler to the end/persistently." (私は[飽くまで]執事ですから)

After freaking out over the possibility of losing r!Ciel, Polaris tells him and Undertaker that if r!Ciel died, "I wouldn't be able to rest, even if I died. I can't imagine where else I could be... for I am a butler, even in death."

私は[死んでも]執事ですから。

This is nearly identical to Sebastian's catchphrase, both saying "For I am a butler," with a single word (and its attached particle) swapped out:

私は(I)[ ]執事(butler)です(am)から(for)。

An obvious problem with this theory is that Polaris is loyal to r!Ciel, while Sebastian is loyal to o!Ciel. An even more obvious problem is that while Polaris is murdering Agni, Sebastian is with o!Ciel elsewhere. This is the point where my implausible theory reaches its peak of absurdity: I believe Polaris is a bizarre doll of Sebastian, specifically Sebastian the butler.

When Undertaker impaled Sebastian on the Campania, he obtained his cinematic record. Like when Grelle cut him with her death-scythe in the Red Butler Arc, Sebastian's record only shows memories of his current body and nothing before it manifested. The sole purpose of the body named Sebastian is to be the best butler and to serve his master. This could also explain the previous master Polaris lost and his insanity over losing another: the last memory of the body the cinematic record was taken from is seeing o!Ciel thrown over a railing and out of reach; Undertaker captured Sebastian's desperation in that moment and used it to create a bizarre doll.