|Thanks to Dial B for Blog for this image|
Two versions of the Green Ghost appeared in Invincible. They were created to be stand-ins for Green Lantern. The second version was solely created to be killed off with the rest of the Guardians of the Globe.
The first one was only mentioned and seen in flashbacks. He was a member of the Guardians of the Globe and friend of Martian Man. He disappeared (possibly dead or retired). A second Green Ghost appeared. This version was an African American inexperienced with using his powers. He was killed by Omni-Man as part of a preparation for an alien invasion.
The first Green Ghost was shown he could fly and generate a green cloud people could stand on. The second Green Ghost could fly and phase through matter. The second one was green swallowing a green gem to gain his powers. It is implied (but not stated) the first one did so as well and had similar powers. When both of them activated their powers, their bodies were covered by an identical green costume.
Invincible volume 2 "Eight is Enough", volume 3 "Perfect Strangers"