General Xaviax is an alien who defeated the Kamen Riders to conquer Ventara before turning his attention to conquer Earth with his monsters gathering humans to set up his master plan of abducting all humans on Earth to be his slaves in rebuilding his home world. Before the start of the series he fought Eubulon, the Advent Master and the creator of the Kamen Riders. They fought to a stalemate which had Eubulon getting vented and sent to Earth instead of the Advent Void. By unknown reasons, he lived through the fight. Some time afterwords, during the time Adam, the original Kamen Rider Dragon Knight, was assigned to patrol Ventara while the others were in a state of suspended animation, he had approached Adam and offered him a deal to betray and vent the other Riders (which the details of the deal is currently unknown) which he does but Len and Kase manage to get away (though at the time Xaviax thought Kase had been vented as well). Regretting his actions Adam secludes himself in an illusion Xaviax created and leaves him there with a Ventaran girl named Sarah. Thus he begins his hunt to eliminate Len, the only Kamen Rider that stands in his way for global domination.
What his world was destroyed by has not been revealed at this point in the series, but it remains the driving force in his conquest of Ventara and his desire to conquer Earth. To that end, he uses his shape-shifting powers to find ideal pawns to aid him as his Kamen Riders, tricking some of them into believing they are doing the greater good or offering them something they desire. He has currently displayed only one human form thus far in the series, but takes on different aliases to suit his needs and recruit Riders to his cause.
In his earlier plots, Xaviax revealed himself as an alien to get James "JTC" Treadmore to his cause.
The first of his disguises is a Preston Family attorney named Walter Conners which he uses to announce Richie Preston's severance from the family fortune and an opportunity to become a bounty hunter of Kamen Riders.
At some point, he managed to get Drew Lansing to his side promising him his part in ruling both worlds.
He later appeared as a mercenary captain named Race Mattock to entice an underground fighter named Grant Staley with the idea of being a great fighter of two worlds.
Then, he appeared as a Battle Club tournament official named Charlie Feathers to blackmail motocross racer Brad Barrett holding the proof of his innocence for a cheating scandal Brad didn't commit. During that time, "Charlie" revealed to Brad that he is Xaviax and gives Brad the option to vent or be vented as Xaviax still has the proof of his innocence.
He later appeared as a government agent named Special Agent Simons, giving the asthmatic marine Chris Ramirez a rare opportunity to serve the United States in secrecy by fighting aliens from Ventara.
He then appeared before the Cho Brothers with a large sum of cash and the chance to pull off the biggest heist of their careers.
When Danny goes rogue after Albert is vented by Kit, he wanted Strike to vent him after Xaviax warned Danny not to go after Kit. However, upon news of Kamen Rider Siren's return, he feels uneasy discovering an even bigger threat than Wing Knight. Even when Strike came back with Danny still alive, he does not act against JTC as he knew that he would be outnumbered and decides to leave Danny's fate up to Strike's judgement in which Strike vents him.
While he has been offering some of the other Riders what they desire for the elimination of Len, one of his primary targets seems to be Kit, as many recent plans involve getting Kit to defect to his side and betraying Wing Knight. So far he has been unsuccessful, even losing his main bait, Frank Taylor. But soon he decides to take a more direct approach after Siren appears and directly reveal himself to Kit. He reveals that he kidnapped his father, framed him for his arrests and then manipulated his father to bring him to the Advent Deck and Dragredder. He states that Kit was the hardest one to recruit as, unlike the other Riders, he had no vice (like Richie's greed) or ambition (like Chris's desire to be a hero). With the training received by Wing Knight as well as fighting his own Riders and gaining Wing Knight's trust, he believes that it is the best time to recruit Kit. With that, he decides to finally make his offer to Kit: vent Wing Knight and Siren and he will revive Frank Taylor, manipulating Kit's love for his father. However when Xaviax realizes that Kit can no longer be swayed, he abandons his plans for him meaning that Kit must now be vented. And to help JTC, he reveals a secret Kamen Rider in emergencies, Kamen Rider Wrath. He reveals that the body is in a coma and physically merges with Wrath's body in order to fight the three Riders who stand in his way. While fused with the body, he takes the appearance of construction worker Vic Frasier (the counterpart of Ventara's Kamen Rider Wrath) instead of his normal form when removing the Rider armor. Xaviax later gives JTC's Advent Deck more power by allowing him to capture two more Advent Beasts and use their former master's abilities as well as fuse them together.
After Strike is vented, Xaviax in Wrath's body manages to attack Dragon Knight and Wing Knight and overpowers both Riders, even in their Survive Modes. He manages to vent Kit, but bails from Vic's body before it disappears to the Advent Void. Xaviax then snatches Kit's Advent Deck and plans to use it against Wing Knight, the last Kamen Rider standing in his way.
His master plan in the series is to kidnap and conquer all of Earth. However, Earth is a Mirror World to Ventara and kidnapping humans one by one is time consuming. So instead, Xaviax has a machine where he can transport a mass amount of humans from Earth to Ventara and use them all as his slaves. But this machine requires specific humans with matching DNA to power this machine. To that end he sends his minions in to kidnap just those people. With Wing Knight, Dragon Knight and other Kamen Riders fighting against him, however, he has to focus on the war against them as well, which delays his plans. But when all but Wing Knight is vented, he returns to his original plan. By planting special devices across the world, it will enable Xaviax to transport all of Earth to Ventara. But after hearing disturbing news of Eubulon's return, he decides to send Adam, the original Dragon Knight, to spy on Len threatening Adam by revealing to Sarah that Adam had betrayed the Riders. After discovering the virus in the system, Xaviax tells Adam to send him a copy of the virus so he could decode and create an anti-virus. However, Adam has continuously failed to inflitrate the Riders, angering Xaviax. He sends monsters to attack Maya to allow Adam the chance to save her as another opportunity to ingratiate himself with the good guys. Upon discovering the virus in the system, Xaviax tells Adam to send him a copy of the virus so he could decode and create an anti-virus. However, Adam has continuously failed to inflitrate the Riders, angering Xaviax. He sends monsters to attack Maya to allow Adam the chance to save her as another opportunity to ingratiate himself with the good guys. When Adam manages to gain their trust and acquires the virus for him, he shuts down a transmitter to make it look like Adam was successful and begins to decode the virus. While he was successful, he feels that Adam may turn his back on him so he disguises himself as Eubulon and fights Adam and berates him during the fight, obviously swaying Adam to betray the other Riders once more. He was able to decode the virus and create the anti-virus with which, he reactivates all the transmitters and begins a countdown to its activation. Before he uses it, he is confronted by Eubulon. He berates him for abandoning his people in Karsh and appears to have the upper hand against him. He is stopped by Kit, Adam, Len and Kase who he refocuses his attention on. When he attempts to finish them off, the other nine Riders arrive. He tells them that he could have been able to give them anything they wanted, but instead they chose to fight against him. But soon all 13 Riders activate their Link Vents and destroy General Xaviax once and for all.