Xero is a mysterious swordsman with Amnesia who is trying to figure out exactly who he is.
Not much is known is known about Xero's History except that he was a honest man who was tricked into giving his body to Dark Matter. He later woke up on Pop Star and met a Demon-like Kirby named Retro and the two became friends.
Xero is in the middle of his quest when he and Retro meet up with Kirby. If one chooses to make him a friend, Xero will set aside his quest and help defeat your enemies, and then later will continue his quest, if he becomes an enemy, he will become 02 once more, making it very difficult to defeat your enemies, if he doesn't become involved in the war, nothing will happen and he will continue his quest.
Xero's Quest tells the whole story of what he does when he wakes up and meets Retro, and his quest to find his memory, The game's story does go through the SKF story, but you do not play through it. The canonical story that happens in SKF is that Xero does not become involved in the war.
Xero displays a vast fighting abilities, but his most prominent is Kenjutsu, the Japanese Art of Swordplay. In an upcoming Game involving Xero, he will show off his transformation abilites.
Before Xero gave his body to Zero, he was a tall, strong man with a metallic body, and his face resembled a skull. During the upcoming Game, Xero can switch between his Zero Form and his Original Form after regaining his memories. In this form he doesn't fight with a sword, as that was something he learned when Zero controlled him, but rather he fought with a Golden Chain (the reason his sword is gold is because Xero's sword is just the chain transformed, Xero's weapons transform with him). Also he can breath Fire in this form.
In the upcoming game, Xero is set on fire in his original form, but instead of melting him, he simply uses the the burning body to master his Pyrokinesis (in his Original Form he can breath fire). In this form he can shoot fireballs, and also make a firechain (His normal chain on fire, and instead of running he shoots forward in a fireball-esque manner.
The Zero Form is the form he's most commonly seen as nowadays. The Zero Form looks alot like Zero's first form, but with a different eye and sword color. In this form he has a golden sword, and fights using that, aswell as powers he gained from Dark Matter. All phases beyond 0 will revert back to Phase 0 if taken enough damage(except Phase 3, which is immortal and turns back to 0 after power drains, and 6, which reverts to phase 5 if half of it's HP is depleted)
Phase 0: Swordsman Form
The most common form, a swordsman, In this form he also can use Orbs of Dark Energy, which he either launches or puts on his hands and punches.
Phase 1: Monster Form
This is more empowered form, but since it hasn't been used for a while, it's power is just as much Phase 0. In this form he fights with the powers of Mariels, Nidoos, and N-Zs. It's appearance is a Swordsman with Spider Legs.
Phase 2: Gooey Form
This form is simply a swordsman form minus the sword, and plus a long tongue. The Gooey phase fights like Gooey.
Phase 3: Dark Form
This form is where Xero becomes Dark Matter, or actually a cloud of them, in this form Xero isn't as powerful as his before forms, but is immortal for the period of time as whenever he takes damage new Dark Matter cloud will replace the one that was destroyed.
Phase 4: Miracle Form
This form Xero turns white and gets large red pores on his hand, with which he can drain the life force of enemies.
Phase 5: Eye Form
A small red orb that appears similar to an eye forms around Xero, and Xero floats around in it shooting Dark Matters at enemies instead of fighting himself.
Phase 6: Cipher Form
A Large white orb forms around the red orb, making it appear that red orb is the "eye" of the white orb, or the "body". Xero can launch himself in the Red orb to attack.
Phase 7: Cross Form
Four Dark Matters appear around him, all with the ability to launch their eyes or throw mini-dark matters at enemies. Each Dark matter has 1/4th of the Power of Xero, so having the Dark Matter doubles the attacks strength.
Phase 8: Oxygen Form
The Body gains wings and a Halo, and the eye turns more elliptical and has a bloody tear coming out of it. The four Dark Matter form with multiple other Dark Matter clouds to form a planet, and the Oxygen Form hides at it's core. In this Form Mariels, Nidoos, and N-Zs all are launched as comets. This Form is only obtained during the final battle when the Queen of Ripple Star Conjoins with the Power Crystal and becomes a Giant Angel and Devon cuts the eye and enters, and the two fuse into the true 02 and have full command of Dark Matter.
In this Form, he takes the appearance of a human swordsman, and has more than one sword. He gains this form in the upcoming game when the enemy sends him back in time, before he became metal, and he becomes a teenager, but with all the skills from him in the future and items from the future, so he becomes teenager, and his sword is turned into katanas.
In this Form, the Chain takes the form of a Golden Tome. In this Form he can perform Magic. He gains this form when the enemy uses magic on him and he can't fight, but he finds a rod and it allows him to manipulate the magic cast on him.