Xaviax

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.

Backstory
General Xaviax is an alien from planet Karsh. His world was destroyed by a civil war between the planet's north (led by Xaviax) and its south. Though the north won, they left the planet in ruins. Under Xaviax's orders, Eubulon created teleportation beacons and went to the planet of Ventara to place them and abduct them. After falling ill and recovering with help from a Ventaran family, Eubulon tried to convince Xaviax that they'd have to find another way to save Karsh. Xaviax, however thought that Eubulon had committed treason and decided to enslave the Ventarans himself. Eubulon warned the leaders of Ventara and created the Kamen Riders to defend the planet, becoming the Advent Master. Eventually, Eubulon and Xaviax fought and their battle ended in a stalemate as both were disabled. Eubulon had been vented and sent to Earth - Xaviax was able to recover more quickly and 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 under the guise of 'Paragon' another Karsh refugee and a supposed friend of Eubulon and convinced Adam that he could unlink the Riders from their decks. Adam took Xaviax to the base to wake up the other Riders in order to give them the good news.

But 'Paragon' turned on them and vented all of the Riders except Len and Kase who woke up in time and escaped, though he thought Kase had been vented. With the Riders out of the way, Xaviax successfully abducted all of Ventara.

Regretting his actions Adam secluded himself in an illusion Xaviax created and left him there with a Ventaran girl named Sarah. Thus he begins his hunt to eliminate Len, the only Kamen Rider that stood in his way for global domination.

Recruiting Riders
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.

The first of his disguises is a Preston Family attorney named Walter Conners which he uses to announce Richie's severance from the family fortune. He makes Richie fight for him by stating that each rider he vents, he will receive one millions dollars.

He later appeared as a mercenary captain named Race Mattock to entice Grant 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 Brad. During that time after Brad found out Grant being vented from Len and Kit, including Xaviax's former henchmen con artist Drew who vented Grant and spills to Brad who exactly "Charlie" is, "Charlie" revealed to Brad that he is Xaviax and gives Brad the option to vent or be vented as Xaviax still has the proof.

He later appeared as a government agent named Special Agent Simons, giving an asthmatic Chris a rare opportunity to serve the United States, albeit in secrecy, until he found out Xaviax's tricks from Kit and Len when they witness Brad being vented by Xaviax's right-hand man JTC, and much worst, Len sense Xaviax's former henchmen Drew being vented shortly by JTC.

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, he wanted Strike to vent him. 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 doesn't act against JTC as he knew that he would be outnumbered and decides to leave Danny's fate up to Strike's judgment in which Strike vents him.

Primary Goal
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 and three of his aliases to Kit. He reveals that he kidnapped his father, framed him for his arrests and then impersonated 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' 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's 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 Vic Frasier, Wrath's counterpart, 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.

Final Plan
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 into 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 Earths population 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.

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 infiltrate 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, this time out of his own free will. He was able to decode the virus and create an anti-virus with which, he reactivates all the transmitters and begins a countdown to its activation.

Before he uses it, he's confronted by Eubulon. He berates him for abandoning his people on Karsh, But Eubulon Justifies it by saying that Xaviax turned the planet into a wasteland through war and he couldn't let him destroy another. He also adds while their time of existence is done, humans still have a chance of survival. Xaviax becomes even angrier at this and says that Karsh and it's people are superior above all others and won't let anything, including him cause their race to die out. As a result the two begin to fight. Xaviax appears to have the upper hand, but 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. In a desperate attempt to corrupt all 13, He tells them that he could have been able to give them anything they wanted, but they chose to kill him once and for all. Soon all 13 Riders activate their Link Vents and destroy General Xaviax for good.