Racer X is secretly Speed Racer's older brother Rex.
Rex Racer (Kenichi Mifune) had a promising career as a racer. Taking one of Pops Racer's cars without his permission, Rex entered a race. Due to his bold and reckless driving, the car was smashed. Pops and Rex argued resulting in Rex running away from home.
Rex later created the identity of Racer X. Racer X soon developed a reputation of being a dangerous racer due to other racers being injured during races. Part of this was due to evil forces gunning for Racer X. Fighting evil, this was a way to protect the lives of his family and also keep watch over his younger brother Speed. Rex had joined the International Secret Police and was forced to keep his identity a secret from his family.
In the original manga, at the end, Racer X revealed himself to his brother Speed, who always suspected Racer X could be his missing brother. His undercover mission was over and he could return to his family. He and Speed would become the world's best racing team.
In the anime, things ended differently. It was meant to be their final meeting, Speed asks Racer X point blank if he was his brother Rex. Rather than give him the answer he had been waiting for so long to hear, Racer X suckerpunches Speed in the gut and knocks him out. He then decides to give up the guise of Racer X and leaves behind his mask for Speed to find. Knowing that the honorable and great racer was his brother, Speed is left somewhat in happiness.
A live-action adaption of Speed Racer written and directed by Larry and Andy Wachowski.