A Doppelganger is a special type of Spectre that appears in Orpheus. Doppelgangers are the counterparts of those of the Spirit Lament, essentially the dark side of a ghost. The sole purpose of a Doppelganger is destroy everything a spirit holds dear in an attempt to bring them to Spectre-dom, or to outright destroy the spirit itself.
Ghosts who have less than five Vitality (which Orpheus classifies as drone, blip, or echo-class spirits) are never aware of their counterpart's existence. Spirits with high Vitality, on the other hand, are not only aware of their dark counterpart, but under the right circumstances can detect if they are nearby.
As the Doppelganger is an almost exact counterpart of a spirit, they have the same abilities, Horrors, manifestation forms, and Stains. However, Doppelgangers use their Spite to power these abilities rather than Vitality. Doppelgangers can be temporarily destroyed, but will always reappear after a few weeks.
When a spirit becomes a Spectre, the Doppelganger and spirit seek each other out and merge to form one being, but with the Doppelganger in charge. When an Orphan-Grinder is made, the split is made again, but what was once the Doppelganger is now a Gemini, basically a powered-up Doppelganger. If the Orphan-Grinder becomes a Spectre a second time, the Gemini merges with it once again, this time obliterating the spirit's original memories and turning it into little more than a puppet.
Doppelgangers are essentially the Shadow from Wraith: The Oblivion, but instead of existing purely in the mind, they have a physical form. It is presumed the chaos of the Sixth Great Maelstrom and its causes may have something to do with the Shadow splitting from the psyche of its wraith, just as it twisted the Shroud into the Stormwall.