Racer X

Racer X is actually Speed Racer's older brother Rex, this story was covered in the episode "Challenge of the Masked Racer (Part 2)". Rex got into an argument with his father after he wrecked the first car Pops built. Rex the chose to leave the family and vowed to become the world's greatest race car driver. He frequently helps Speed out of trouble from people who would want to hurt Speed because of his driving skills. Rex does this under the as an "agent" working with law enforcement.

Racer X drives a yellow race car with the number 9 known as the shooting star

Casting
In the original series he was voiced by Peter Fernandez, in the Speed Racer Film, Racer X is portrayed by Matthew Fox.

Alternate Stories
In the 2008 movie and the Speed Racer X television show, Racer X is be thought dead after the car accident instead of leaving the family.