Racer X (initially The Masked Racer) is a frequent recurring character in Speed Racer's adventures. Racer X drives the number 9 car, the Shooting Star.
Racer X is a heroic, mysterious, flamboyant, selfless, sympathetic and often brooding soldier of fortune. His true identity is Rex Racer (三船 研一, Mifune Ken'ichi), the eldest son of Mom and Pops Racer and the older brother of Speed and Spritle. His 'Racer X' moniker is actually a play on his own first name Rex. Years before the start of the series, when Rex was 18 years old (Speed's current age), he had a falling out with Pops after wrecking the Mach 1 (the first car Pops ever built) at the end of his first race as a professional driver (seen in flashbacks throughout the series).
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 an 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.
Throughout the course of Speed Racer, Racer X's identity is periodically revealed to the audience as Rex, while Speed is unaware of it. Speed starts to suspect that Racer X might, in fact, be his estranged older brother in the "Challenge of the Masked Racer" saga (episodes 3 and 4). Speed finds out that Racer X is his long lost brother near the end of the series in "The Trick Race".
It was acknowledged by both Pops and Speed over the years that Racer X was the superior driver of the two, and the greatest driver that they had ever seen, but Speed always vowed to defeat Racer X as the two vigorously competed. 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.
In several stories Racer X is seen as an international spy or secret agent and is often responsible for thwarting evil corporate schemes. In this persona, he is known as Agent 9 (a reference to his car number) and is often portrayed wearing a white linen suit with Panama hat and often wrap around sunglasses. The Shooting Star has a communication device which allows him to report to his organisations headquarters in Paris. 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 whether the Inspector has a dossier on Racer X. In the episode "The Secret Invaders (Part 2)" Speed also speculates the possibility of Racer X being a secret agent.
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.
Racer X appeared in the live-action film and was portrayed by Matthew Fox. He first appeared crashing into Cruncher Block's truck to save Taejo from getting tortured. He later joined Taejo's team in the Crucible race.
The relationship between Rex and young Speed is built considerably with Speed looking up 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, but Racer X gives the former advice to figure out his own driving and to not let the racing world change the way it is. 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 afterward 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 made a mistake by leaving his family, Rex responds to him saying that "If I did, it's a mistake I'll have to live with.", meaning Rex has to continue hiding his identity in order to keep his brother safe.
- Racer X was a 1980s heavy metal band founded by future Mr. Big guitarist Paul Gilbert.
- Dallas band Slow Roosevelt recorded the song "Racer X" on their album Weightless.
- The band Big Black's 1984 EP is Racer-X. The title track is full of Speed Racer references.
- The cartoon Dexter's Laboratory has a Speed Racer parody episode, "Mock 5". In it, the main character Dexter's older sister Dee Dee portrays both Racer X and Spritle.
- The comic strip Liberty Meadows has a story line around a dachshund ("sausage dog") race. One of the canine competitors is named Wiener X and wears a mask like Racer X.
- In the animated TV show Fetch with Ruff Ruffman (Season 4, Episode 1), Scruff Ruffman was Racer X in a Luge Race.