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Speed was often suspicious of Racer X's identity and motives because Racer X would repeatedly, and inexplicably, sacrifice winning races to protect Speed from drivers and others who tried to harm him. The assistance from Racer X nearly always led to Speed winning races, while Racer X came in second place. Racer X always left the scene unnoticed, receding into his secret life. It was not until the episode "Challenge of the Masked Racer Part 2" that fans of the show finally got to see the face of Racer X. His face was shown a second time near the end of the series in "The Trick Race".

In that episode, Speed finally gets a chance to ask Racer X if he is his older brother--only to be interrupted by a punch in the stomach. As Speed lies unconscious, Captain Terror, of the Car Acrobatic Team, soon recognizes the resemblance of Rex under the mask of Racer X. Racer X confesses that he is truly Rex. He then vows to quit automobile racing and becomes a full-time international secret agent instead. As he walks away, he takes off his mask and he tells the unconscious Speed that he will look after him wherever he might go. When Speed wakes up, he finds Racer X's mask on the ground beside him. He is finally convinced that Racer X is in fact his older brother.

Pops had told Rex prior to the race that Rex was not yet prepared to compete at the professional racing level. With less than one lap to go in his first major race, Rex was leading and cruising toward victory, but lost control of the car and wrecked it. Pops exploded with anger and berated Rex. In even more enraged response, Rex fled the family and exiled himself while vowing to become the world's greatest race car driver. During his exile, Rex assumed the mysterious, Racer X (a.k.a. Masked Racer) identity, to pursue his racing career. In several stories Racer X is seen as an international spy or agent and is often responsible for thwarting evil globalist or corporate schemes.

In this persona, he is often portrayed wearing a white linen suit, Panama hat and wrap around sunglasses. Racer X's agent persona is clearly identified with the Western Powers of the Cold War and is inspired by John Heath. He also seems at times to be working in cooperation with Inspector Detector, though it is not clear if he actually works for the same organization. Inspector Detector also seems to know about Racer X's identity though again it is unclear if this knowledge is mutually shared through the relationship or that the Inspector has a dossier on X. In the 2008 film adaptation of the manga and anime, Rex Racer is portrayed by Scott Porter while Racer X is portrayed by Matthew Fox.

The relationship between Rex and young Speed is built considerably with Speed looking on to Rex as a role model and learning to drive from his older brother. Rex drives the Mach 5 in the film and gives it to Speed when he leaves Pops' racing team to drive for corporate interests. As a result, in this adaptation Rex does not destroy the car he drove for Pops (the Mach 4) and is portrayed as a reckless and cheating corporate driver responsible for numerous crashes before being killed at the entrance of the Ice Caves in the Casa Cristo cross country race. When Racer X appears in the film, he has been working as a gun for hire and is feared by many drivers, sponsors, and corporate interests.

Racer X works with Inspector Detector tracking the corruption that has taken over racing, and contracts Speed to work with him and Taejo Togokhan (a key witness in the case against Royalton's corruption) in the Casa Cristo that season. Speed begins to suspect that Racer X is in fact Rex, noting that the masked racer appeared exactly two years after his brother died in "The Crucible" and exhibits the same driving styles. After Taejo Togokhan reneges on the proposed takedown of Royalton Industries after Casa Cristo, Speed recklessly races the Mach 5 at the track where Rex took him to drive when he was younger.

Racer X appears and Speed confronts him about his suspicions, especially due to the two's similar driving styles and the fact that Racer X found him at that particular race track. Removing his mask, Speed is depressed to learn that Racer X is not his brother. However, as the film closes the truth about Rex and his work before his "death" is revealed and he is shown faking his death in the caves, detonating his car and undergoing cosmetic surgery afterwards to alter his appearance in order to take on the corruption without bringing harm to his family. In the end, as in all his other versions, Racer X is Rex Racer.

When Inspector Detector asks Rex whether he ever regrets leaving his family, Rex responds to him saying that "If I did, it's a mistake I'll have to live with." From: Wikipedia