Quite literally, the haunting soul of Nexus. In most of Nexus' character arcs, he dies at some point in the story, either due to defeat, the natural passage of time, or intentionally sacrificing his mortal form for more power. Depends on which version of the guy we're talking about.
Nexus' Wraith was based off the original drawing of Nexus from an old high school sketchbook. Originally, just a warlock, always drawn with his face half in shadow and glowing eyes. This version isn't the same, just removing the limbs visible from his coat and no ears - and a scar, a reminder of an injury suffered before death that haunts (pun intended) him in the afterlife. In his incarnation in Sartris's story, it's also meant to mirror Sartris's scar.
Character Profile: Wraith