This is a tiny life-form molded out of magical frozen flame. When a familiar holds it, it forms itself into a miniature replica of the familiar. As the doppelgänger mimics the familiar's movements, it transmutes the experience it gains to the familiar. Unfortunately, the doppelgänger's mute mimicry tends to enrage monsters. No one's sure why.

The upshot of this is, of course, that your familiar gains more experience per combat, and fights angrier monsters, when it has the doppelgänger as its equipment. That such an insanely useful creature should exist has been widely regarded as the definitive argument against the existence of God.