X.A.N.A.
X.A.N.A. was originally a simple program created by the scientist, Waldo Schaeffer, under the alias of Franz Hopper. Its design was based on the principles of a multi-agent system and a computer virus for the purpose of destroying Project Carthage, which Waldo defected from due to the threat it posed. X.A.N.A. was meant to have simple intelligence to be easier to control, but it grew stronger and more intelligent with each return to the past, slowly but surely increasing X.A.N.A.'s awareness until it became autonomous and self-aware.
Known Details
1 / Creation
XANA was created by Waldo Schaeffer, then under the alias Franz Hopper, in direct opposition to Project Carthage. The intention was for XANA to oppose and stop the project before it could its full potential could be realized.
2 / Returns to the Past
To give himself time to work out the kinks with XANA and the supercomputer, Hopper launched a program called "Return to the Past" (RTTP), which did as the name implied. This allowed Hopper to relive the same day 2,546 times, which also doubled XANA's computing power each time effectively making him both smarter and stronger with each RTTP launched.
3 / Sentience
When XANA gained sentience, it began to view both Hopper and his daughter Aelita as threats. Hopper did not realized the extent of its new intelligence until he virtualized both himself and his daughter to Lyoko where XANA attacked both on sight. Hopper was forced to shut down the supercomputer in order to stop his creation from causing any more damage.
4 / Destruction
When Jeremy Belpois executed his multi-agent system, XANA had to act quickly to save himself from destruction. Luckily, he didn't need to look far for sanctuary. The scientist Alan Meyer saved XANA himself, isolating it to a singular server and began his work to "perfect" it. Still, cut off from its previous massive source of power, XANA is essentially back to square one.
Surface Sector Monsters
"Hi, my little monster friends. Did you miss me?"
Monsters are virtual creatures created by X.A.N.A. to serve as its minions in virtual worlds. They are directly virtualized into Lyoko and are primarily used to fight its enemies, mainly the Lyoko Warriors. According to codelyoko.fr, X.A.N.A. uses a base template which is used to make it easier to create new monsters.
Kankerlats
These monsters are X.A.N.A.'s weakest creatures, but arguably the most versatile. They usually attack in large swarms. They are much faster than Bloks, and are fairly hard to hit because of their small stature. Sometimes they attack from surprise.
These are Yumi's least favorite monster
Ulrich has killed the most, 22 in total
Hornets
These are the first monsters that are capable of flight and are one of the few to have more than one attack. They can fire rapid-fire lasers from their stingers and spray poisonous acid from their heads. Cutting off their wings makes them completely immobile.
They utilize their speed to do attack runs on targets, quickly flying in with a barrage of lasers and leaving just as fast.
Yumi and Odd are tied for most kills with 25 each.
Bloks
They are one of X.A.N.A.'s most used monsters. Bloks are cube-shaped turrets with six crab-like legs and have three different attacks: standard laser, fire ring, and freeze ray (which can immobilize its target in ice for exactly three minutes if it doesn't cost the target the remainder of their life points as is has done with Ulrich on multiple occasions). However, with an eye on each side of their 'heads' (four in all), their major weakness is that hitting any eye can destroy them and turning them upside down makes them completely immobile. When they are stacked on top of each other like a wall, they shoot an absolutely devastating grand laser.
These are Odds favorite monsters
Ulrich has killed the most, 21 in total
Krabs
They are arguably the tallest of monsters, the most versatile, and generally found in groups of two or three. They can use their legs to stab their enemies, and fire strong lasers from their eyes. In The Chips Are Down and Wreck Room, they demonstrated the ability to project pulse beams from underneath them. This is the only monster that can be destroyed by cutting off its legs.
Krabs greatly resemble a monster found in the video game Ultimate Blaster.
Yumi has killed the most, 22 in total
Megatanks
These are one of X.A.N.A.'s most powerful monsters. They are huge, armored spheres with indestructible armored outer-casing. They attack with a powerful elliptical laser that can run straight in all vertical directions. To do this attack, it must expose its support ring the laser is fired out of where the Eye is under its armor, making it vulnerable in that moment.
These are Ulrich's least favorite monster
Odd has killed the most, 8 in total
Tarantulas
They were first introduced in New Order, where they were depicted as X.A.N.A.'s deadliest monsters due to having more life points than the others and their lasers doing twice as much damage. They are excellent shooters and are very durable. They can fire ray beams from the tips of their front legs that take twenty life-points per impact.
It has more life points than Krabs, Hornets, Kankrelats, and Bloks.
Odd has killed the most, 14 in total
Sector 5 Monsters
"Hi, my little monster friends. Did you miss me?"
These monsters are either confined to Sector 5 or were originally created here. No other monsters can be virtualized to Sector 5 but these two and the Schyphozoa.
Creepers
They are exclusively found in Sector Five. Creepers are lethal snake-like monsters that attack by firing lasers from their mouths and make up for having less life points than other monsters by having much more powerful lasers, taking forty life points per hit.
Creepers were never shown outside of Sector Five. Unlike the Mantas and Scyphozoa, both natives from Sector Five, they are incapable of leaving. This is possibly due to it being the only Sector Five native unable to fly, or its weak defenses make it negligible in other sectors.
Odd has killed the most, 28 in total
Mantas
The strongest flying monsters. They are large manta ray-like creatures with strong lasers and hatch flying mines. They can fly but are slower than Hornets. In Cold War, X.A.N.A. shows that it grew powerful enough to send them to Lyoko's surface sectors. These are the only monsters with colored variations, as William had his own unique black manta.
Mantas are the only monsters that can travel outside the Celestial Dome in Sector Five.
Odd has killed the most, 17 in total
Unique Monsters
"Hi, my little monster friends. Did you miss me?"
These monsters can only be virtualized one at a time- no variants have ever existed at the same time.
Schyphozoa
One of X.A.N.A.'s most unique monsters. It has the appearance of a giant jellyfish and is almost impossible to destroy. It is the only monster that lacks offensive abilities but makes up for it by being able to siphon digital memory from a victim with its tentacles. It can also implant X.A.N.A.'s virus code into their memory, controlling their minds to do its will. Aelita has been its main target, but it has attacked Yumi once and William twice.
The Scyphozoa is most likely XANA's version of a phishing program, a type of Malware specifically designed to harvest data for further manipulation.
Jeremy/Aelita are the only people who have destroyed this monster.
Kolossus
X.A.N.A.'s largest and ultimate monster. Created by drawing computing power from all of the hundreds of Replikas that it has established all over the Internet. It has the appearance of a giant lava elemental being, with a giant blade replacing its left hand. Its body is indestructible and one attack from it results in instant devirtualization. The only way to destroy it was to simultaneously attack the Eye of X.A.N.A. on its head and its left sword arm.
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NOTE: XANA does not currently possess the resources to virtualize this monster.
Unlike normal Monster Virtualization, the Kolossus can only be virtualized by drawing energy from all of the Replikas.
Fun fact! This monster has never been devitalized but Ulrich is the only one who has defeated it.