X is one of four (if you include Ryuuji) playable characters in the Crossover X series, as well as the namesake. He is a purple dragon, with a similar body shape to Dragonhunter's first form. He has a black mark under his right eye. He's very good as your power player, because he is the strongest playable character, with a high Attack stat. He's low on Defense, however, and is of average speed. In Multiplayer mode, the third player plays as him, even if he's still locked in Single-player.
X is locked in Single-player at first. To unlock him, you must find the Violet Amulet in the "Lost Grove", a hidden area in the second Get to the End stage on Ghiria. Either that or you can unlock him by racking up fifty enemy KOs. Both take a while, but the former is far safer because no enemies appear in the Grove.