After beating some of Mr. X's minions, Mr. X will appear to spontaneously combust before revealing his true identity. Robo X slides around the arena, stopping to fire a machine gun on his hand or launch a homing missile. If the player gets within melee range he will throw a quick series of consecutive punches. If a player grabs him and hesitates before throwing, after a moment he'll electrify himself, doing the player some damager. After attacking the player, he'll stop to laugh, giving the player a chance to attack him.
After his defeat, in Easy mode in the English version of this game, he'll tell the player to play Normal or Hard mode. If the player does defeat him in Normal or Hard mode, the player will have access to Stage 6. On the Japanese version the player can access the 6th stage regardless of difficulty.
Robo X, now known as Robot Xes, reappear in Project X Zone 2, being the only Syndicate minion other than Break to be present in the game. They, like the Axel imposter, are brainwashed by Bison's Psycho Power, fighting Shinra and their allies starting from Chapter 11 and ending at Chapter 40, where they, like Break, are free from the madman's influence.
Robo X reappears in Streets of Rage 4, through the Mr. X Nightmare DLC as an enemy in the retro stages of the games survival mode.
Robo X appears as an enemy in the good ending version of stage 7, under the name Mr.A-Z or simply Robot in the English version.