Gayle Marsh had a normal childhood until the age of twelve, when she developed extraordinary psychokinetic powers. After her parents died, she was tutored by the scientist Daniel Pendergast, who prepared her to fight a vague threat he called "the Decay". As a college student, Gayle took on the costumed alias of Psi, and repeatedly battled Power Girl, whom she had mistaken for an ally of the Decay. After realizing that "Decay" was actually Pendergast himself, a living manifestation of his own fears, she put a stop to both Pendergast and herself. Trying to atone for the bad she had done, Psi later joined the Suicide Squad (II) for a mission in Russia, where she was tragically killed by the Rocket Red Brigade.
[Note: The theory that Psi fought Power Girl post-Crisis has not been confirmed on-panel, but it seems reasonable since other enemies of the pre-Crisis Supergirl (such as Reactron) have been retconned into Power Girl foes.]