codelyokostorys
It is a virus program that has evolved into a dangerous entity which inhabits and controls the Supercomputer that holds the virtual world of Lyoko. By activating Towers, XANA takes actions and launches attacks in the real world, and can program monsters, alterations, and traps on Lyoko to fight its enemies.
After its supposed destruction, XANA managed to miraculously revive itself through infecting the unknown Cortex Replika, becoming even stronger and more crafty than before in Code Lyoko: Evolution.