The Kolossus (fr: Kolosse) is the largest and most powerful monster created by Xana, larger than the towers on Lyoko and strong enough to push them over barehanded. The Kolossus is created using the combined power of hundreds of supercomputers that Xana has control over. It resembles a golem made of hardened lava, with the molten kind flowing through its body like blood. Its head somewhat resembles an insect, with two loose mandibles hanging from its chin. Several dreadlock-like extensions also come out from the top. The Kolossus has five eyes of Xana on its body: there is one on its head, one centered on its sword arm, and three smaller ones clustered around the latter. Hitting either eye has no effect on the creature, but hitting both large eyes destroys it. When it dies, the lava portions of its body harden, leaving a frozen corpse behind. It is believed that the Kolossus' corpse disappears after a short period of time, like the other monsters.
The Kolossus has no visible laser weapon. Instead, it causes devirtualization by either stepping on its foes, landing on them, or swiping at its enemies with its giant sword-like hand. The Kolossus uses no strategy in battle; its only function is to devirtualize the warriors and to destroy the Skid. While powerful, its size prevents it from attacking rapidly, so other monsters are occasionally deployed alongside it to finish off any warriors it fails to devirtualize. Also, due to its immense size and space, its deployment is limited only to a battlefield large enough to accommodate it — namely the Ice Sector.
- The Kolossus is the largest of XANA's monsters, and it is the most destructive.
- Because its the largest, it only appeared in the Ice Sector due to the sectors large plains. It can probably appear in the Desert Sector but not the Mountain Sector and the Forest Sector because of the narrow paths.
- It is the only monster that does not explode when it is defeated, but it is believed that it disappears some time after it's defeat.
- The Kolossus is the only monster that has been shown to tilt or even push over Towers and when it touches a Tower, the Tower glows red as if XANA has control over it.
- The Kolossus has only appeared in the Ice Sector.
- Some fans believe that the Kolossus was created from the fire of the Volcano sector given its appearance.
- The Kolossus has been created from XANA drawing energy from all of the replika's