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. 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. 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 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, "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. 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 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 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.

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 that Adam had betrayed the Riders.

Xaviax is portrayed by William O'Leary.