Pulse, aka Gus, is a former Marine with the mutant ability to disrupt power systems. He can not only disrupt electrical systems but mutant abilities as well within a two-block radius.
Gus and John Proudstar were friends, but Gus was apparently killed during a break-in at a mutant facility. He turned up two years later working for Sentinel Services, with a tattoo on his wrist. Presumably it has something to do with his apparent betrayal.
