Teen Titans: Animated Series
Season 1, Episode 9: Masks
In this episode, Robin becomes obsessed with learning the plans of Slade.
It is then discovered that Slade desires some microchips and, to the surprise of the Titans, a new villain named Red X, who seems to know all their weaknesses, gets these microchips first and Robin always arrives too late to help his teammates.
Eventually Red X makes a deal with Slade and, after the confusing conclusion, it is revealed that Slade knew the identity of Red X all along: it was Robin! Robin had become Red X so that he could gain Slade's trust and then defeat his sworn enemy.
Yet, Robin decides to retire his Red X costume and the Titans never expect to see their admirable foe again.
Season 3, Episode 2: X
Someone breaks into Titans Tower to steal something from Robin, an old uniform...and so it is that the Titans go to fight someone who has orchestrated numerous robberies only to find themselves bested by someone they never thought to see again, Red X!
The only difference is that this time Robin is defeated along with them. This leads to the Titans (mainly Beast Boy) making up numerous theories of how it would be possible that Robin is responsible. This includes many theories that point to individuals like Jason Todd, evil versions of Robin, a future version of Nightwing, and DNA-spliced cyber-monkeys.
Eventually this theory is given up, but Professor Chang captures the other Titans in an attempt to use the element xenothium for his evil purposes without any resistance.
It is then that Red X is seen as an anti-hero when Red X and Robin work together, with their similar fighting styles and attitudes, to take down Professor Chang.
After this victory, Red X disappears. But it is suspected by Robin that he/she will return.
Season 5, Episode 2: Homecoming Part 2
Red X only makes a cameo appearance in this episode, but it is defining. He/she seems to have been drafted as a member of the Brotherhood of Evil, whose goal is to eliminate the Teen Titans.
So it is that one wonders what this mysterious character's motives could be, and they are never truly explained.
Season 5, Episode 9: Revved Up!
In this episode Ding Dong Daddy steals Robin's most prized possession and organizes a race, the victor will win the item.
As the match commences, it is evident that none of the Titans stand a chance except for Robin, and he still has to beat Ding Dong Daddy. Then it is that a dozen or so other super-villains join the race, with hopes to win their foe's most prized possession.
While the race has been going on for some time, the Titans try to help Robin in every way possible (mostly out of curiosity, and wanting to know what's so special about Robin's stolen item).
Suddenly from the starting line, another racer appears. A black streak easily dominates the race and, as it slows and catches up to the competition, it is revealed to be Red X.
Red X and Robin fight for second place in the race, and when Ding Dong Daddy almost kills Red X with his tricks, Robin saves him. Red X then asks Robin how much he wants the prize and, after receiving Robin's answer, Red X jumps off Robin's motorcycle and leaves Robin in second place.
Red X then proceeds to effortlessly defeat all the super-villains tailing Robin within mere seconds. He then informs Raven and Starfire that now he and Robin are even, implying there's some sort of history that he shares with Robin.
Red X is mentioned in Terror Titans; he was someone that was being searched for to be used in the Dark Side Club's fighting arena.
Though in the Teen Titans Animated Series, the original one to use the persona of Red X was Robin when he was trying to gain the confidence of Slade.
The suit was stolen and has now become a character whose gender, name and even species are completely unknown.
The character shares many similarities with Robin and, though Red X fights the Teen Titans, his motives seem to place him as an anti-hero. Many fans believe Red X's identity to be Jason Todd, but this is unconfirmed in the original series. In the DC Nation special "Red X Unmasked", he is initially revealed to be Todd, before humorously showing that to be the first in a series of masks. After Red X escapes, with his identity still unknown, Beast Boy says: "I still think it's Jason Todd."
Though Red X shares the same stealth and martial artist skills as the Animated Series version of Robin, Red X receives the majority of his/her powers from the suit he/she stole from Robin. It is powered by the unstable fictional element, Xenothium.
The power suit contains a belt that allows its user to cloak or teleport themselves.
Also, red X-shaped projectiles are used that can have a variety of effects and are able to do almost anything imaginable. They have been used as shuriken, close-combat blades, restraints through adhesion or movement restriction, doorways, electro-disruptors or electrocution-causing devices. His X`s are so tough that Starfire can't even break them.