Racer X is Rex Racer - Speed Racer's brother.
In the movie, he fakes his death to protect his family from harm from the big industrial racing corporation. He keeps his identity a secret not only with a mask and sunglasses, but also got a surgical "makeover" on his face to change his identity. He later on in the movie reveals his face to Speed, but Speed doesn't recognize him due to the surgery.
Since Super Smash Brothers Brawl just recently came out, you'll probably be thinking to yourself, "Damn, that guy reminds me of Captain Falcon!
As soon as Racer X stepped out of his car to help that Asian guy, I thought to myself "HOLY SHIT, it's Captain Falcon!"