Doppelganger

You can tell it's Skye's second personality, which hides behind his eye patch and torments him in his own "dreams". It's actually a powerful entity that's better kept under control.

The Doppelganger was intended to be Skye's "true form", but that's not really like that, as his final appearance is a hideous strange monster that has become a separate entity. He's kind of like Skye's own monster, repressed with a manifestation of the madness that floats around Skye's mind, behind his cool exterior.

He lives in the Monochrome Clock Works, a world that Skye's mind often goes to when he loses consciousness. He often has to deal with Doppel's taunts about how weak Skye is, makes him relive the past and tries to convince him that Doppel is free, and that "together" they can tear the world apart.

Doppelganger is Skye, but at the same he isn't - hence his name.