The Rival Flash is Dr. Edward Clariss, a former professor at the university attended by the Golden Age Flash, Jay Garrick. He believed he had recreated the formula that gave Garrick his speed, which he calls "Velocity 9." Bitter at the rejection of his claims by the scientific community, Clariss became a criminal, wearing a darker version of the Flash's outfit. His version of the formula, however, proved to be temporary, and he was defeated when it was expended. In JSA #16 (November 2000) it was revealed that the Rival battled the Flash several months after his defeat; in doing so he reached lightspeed and vanished into the Speed Force.
50 years after his disappearance, shortly after the reformation of the Justice Society of America, Clariss was retrieved from the Speed Force (which had been revealed as the Valhalla of fallen speedsters, good or evil, as well as their source of power) by Johnny Sorrow, who invited Clariss to join the new Injustice Society. The Rival, driven to insanity by his time in the Speed Force, raced across the country on a super-speed killing spree. The Flash realized that the Rival's path across the country spelt out Clariss' name, and that the final murder would be Jay's wife Joan. The Flash managed to absorb the Rival's speed before he could kill Joan and he was reabsorbed into the Speed Force.
The Rival returns in Impulse #88 (September 2002), posing as Joan Garrick's doctor, Dr. Lateris. Now pure speed energy, he possesses Garrick's fellow Golden Age speedster Max Mercury. They both were absorbed into the Speed Force. Max Mercury has since returned, but the Rival has not. He has not been seen since.
There is another Golden Age Reverse Flash, a robot who wears a reversed-color version of Garrick's costume. The robot's only appearance was in one panel in The Flash vol. 2, #134 (February 1998), in which he is easily defeated by Garrick.