Racer X

Racer X is actually Speed Racer's older brother Rex. (This was revealed in the episode "Challenge of the Masked Racer"). Rex got into an argument with his father after he wrecked the first car Pops built. Rex 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 as an "agent" working with law enforcement.

Racer X drives a yellow race car with the number 9 known as the "Shooting Star".

Character traits
Racer X is the most misunderstood racer on the formula one circuit. Both feared and revered, he inspires rumors that whenever he races crashes are sure to happen. A face mask shrouds his face adding to his mystique. Racer X frequently challenges Speed to races of the track, but during real races he helps him.

Relationships
Unbeknownst to the rest of the Racer family, Racer X is Rex Racer, Speed Racer's older brother. He also has ties to Taejo Togokhan: he, Taejo, and Speed work to together in the final race to stop the Tatsunoko company mobsters.

Behind the scenes
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Trivia
In every episode (that Racer X appears in), the narrator confirms that Racer X is Speed's older brother who went missing.

In the original anime, Racer X's white shirt had an "M" on it, just like the Mach 5 symbol, making it a little obvious that he was somehow connected to the Racers. In the movie, this is changed to a black suit with another "X".

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

Alternate Stories
In the both the 2008 movie and the original Speed Racer television show, Racer X is thought to be dead after the car accident instead of leaving the family. In the movie, during a flashback, it is shown that he destroyed the car using explosives and he had re-constructive surgery on his face and voice, in order to fool Speed Racer into think Racer X is someone else, although Speed Racer knows that Racer X races just like Rex Racer.