Gastly is one of the five Pokemon in Death Reaper JOE's team, and comes from the Alternate Universe series. He is a relatively minor character, who only shows up when the situation of the comic calls for it. Although he has an immunity to physical attacks, he is the weakest of JOE's team in terms of power. Instead, he is used as a support by affecting the status of his opponents, such as poisoning them or putting them to sleep, and also by creating a haze to make it hard for his opponents to see.
He doesn't take to losing very well, and he doesn't like to follow orders much, either. Overall he shows no enthusiasm towards Pokemon Battles and instead would rather goof off, loaf around or hang out with his brother, Haunter. For this reason, he's not very dependable to JOE, and because of his frequent bouts of underworld mischief (in between being lazy), he even may be more trouble than he's worth.