Born in Chicago, Gayle Marsh had a fairly normal and undistinguished childhood. At the age of 12, however, the child began to display the signs of extraordinary psychic power. Her parents were frightened by the powers that Gayle was displaying and so they consulted the family doctor. He recommended that Gayle be turned over to the more experienced hands at the Institute of Higher Psychokinetic Study, a Chicago based research facility specializing in the study of such extra sensory phenomena.
The researchers at the institute were excited at the prospect of examining Gayle, finding her telekinetic powers to be some of the most powerful ever recorded. One researcher, Daniel Penderghast, seemed to find Gayle's powers particularly fascinating and a friendship quickly developed between him and his young subject.
Initially Gayle was only meant to be at the Institute for a week of so, but during her stay there, her parents were killed in a road accident. As Gayle had no other family to take her in, Pendeghast petitioned the courts and was made her legal guardian.
Under Penderghast's supervision, Gayle was taught how to use and develop her great powers. Penderghast had plans for Gayle to help him fight a grave menace he believed held the world in its sway. He saw something called "decay" affecting everything and everybody - stripping the world of order and anything that was good. In Gayle he saw a warrior capable of fighting these forces of decay.
By the time Gayle entered college at Chicago's Lake Shore University, her powers had reached their peak and Penderghast decided that it was time for the battle with decay to be joined. Yet Gayle was hesitant to take up the fight, fearing the emotional backlash from her victims would be too painful for her to handle, due to her being a "sensitive" - a reciever of emotions from others.
Gayle was unleashed upon Chicago after a frightening encounter on the campus of Lake Shore University, when she sensed the presence and power of Supergirl and mistook her as an ally of the decay.
Working under the name of Psi, Gayle displayed a startlingly high degree of power in her battles with Supergirl, fighting her to a standstill. However, Psi's reluctance to kill was obvious and Supergirl was able to take advantage of that by playing on Psi's fears. Trapped between the the telepathic urgings of Penderghast and the logical arguments of Supergirl, Psi's fragile emotional state collapsed and she fled in fear from her mission.
Disappointed by her showing and believing she had gone over to the side of decay, Penderghast saw it as his duty to kill Gayle. Though confused, Gayle was still able to defend herself. The result being that Penderghast was turned into the living manifestation of his own worst fears, becoming Decay, a creature whose very touch could decay and rot any and all matter.
Psi later joined the Suicide Squad, and she was killed by the Rocket Brigade in Russia.