Anti-Lad

Anti-Lad was a native of the 75th Century, who discovered a chronological inaccuracy when he saw Superboy's repeated failure to join the Legion of Super-Heroes in a time-scanner. Determined to preserve history, he travelled back and auditioned for Legion membership as "Anti-Lad," who could turn the Legionnaires' powers back on themselves. His ruse was discovered by Brainiac 5. Anti-Lad then tricked Brainiac into believing that he had sabotaged Superboy's auditions, prompting them to give him a more lenient chance. Anti-Lad disappeared after his "misdeed" was "discovered," and no memory of his existence- save a photo of him with the team- remained.

Post-Crisis

After the Crisis on Infinite Earths, he likely ceased to exist; he definitely ceased to exist after Zero Hour, when the Legion's history started over. He likely can be found in one of the many hypertimelines, however.