Eubulon

Eubulon known as the "Advent Master", is the creator of the Kamen Riders from the Planet Karsh.

Backstory
He once worked with General Xaviax to create a machine to transport a large amount of people from other planets starting with Ventara. He had a change of heart in this plan and created the Kamen Riders to destroy the machine. He created a Kamen Rider just for himself and has the power to change the DNA sequence of the Rider Decks so anyone can use it. He fought Xaviax to a draw and ended up being found in Roswell on Earth. He warned the No-men of Xaviax upon his arrival but was injured badly and was unable to tell them anything else.

Reawakening
60 Years Later, Len was brought in to reactivate Eubulon. Len had informed him about Adam's betrayal and of Xaviax's activities. With Xaviax on the move, he reassigns the Siren deck to Maya as she helps Len stall Xaviax as Eubulon goes to retrieve the Riders in the Advent Void. He manages to bring the Ventaran Torque, Strike and Axe (named Chance, Price and Hunt respectively) as he and Len explain their current situation to them. He gives them a device that will upload a virus in Xaviax's transportation devices as they will need some of them to retrieve the people of Ventara. Eubulon goes back to the Advent Void to retrieve other Riders, including Kase and Kit. When he brings them out of the Advent Void, he gives Kit the Onyx deck to help out Len, Chance and Price. After returning to the No-Men's base, he begins repairing the key in order for it to fully function again, before sensing Maya in danger and sending Kit to assist her.

When he senses Kit and Adam fighting each other, he interrupts their battle in order to hear out Adam's side of the story and prevent Kit from giving into anger. He brings Adam back to the base to hear him out, but the other Riders are not interested. Len leaves the base but Eubulon follows him and manages to have the others trust Adam, if only slightly. He goes with Price and Hunt to locate another transmitter but have a difficult time searching for it. After being ambushed by monsters Eubulon retreats when Hunt gets injured. He begins to heal Hunt when they return to base.

Eubulon, while meditating, is sought out by Kit and Maya who ask him of the origin of the Kamen Riders. He tells them that he himself is an alien as well as someone who worked under Xaviax. He tells the history of Karsh that it is a war-torn world and Eubulon was enlisted to create a transportation device to send a mass amount of people as slaves to rebuild their world. He explains that he was taken in by a Ventaran family and decided to rebel against Xaviax not wanting to enslave a peaceful planet. Thus he gave the Ventarans the Rider Armor to defend themselves and the Ventarans selected twelve of their very best to take up the mantle of Kamen Rider.

Later, Eubulon discovers the reactivated transmitters. When Adam and Len return, Adam tells what happened to him and Eubulon reassures him that it was a trap set up by Xaviax. He sends the Riders along with Trent and Maya to Xaviax's base and hands them each a Link Vent card which, when used by all 13 Kamen Riders, will combine all 13 Final Vents and unleash it into one devastating attack. When the Riders destroy the shield generator, Eubulon manages to stop Xaviax right before he activates his transmitters. This leads to a battle between the two of them with Xaviax gaining the upper hand. He is saved by the two dragon Riders who, along with Len and Kase, distract Xaviax long enough for Eubulon to bring the remaining six Riders from the Advent Void. Soon enough, all 13 Kamen Riders use their Link Vents and destroy Xaviax once and for all. Eubulon states that their work is far from finished and together with the other Riders, manages to bring back the people of Ventara.

During the Epilogue, he separates and retires Adam from the Dragon Knight deck and gives it to Kit Taylor, who has officially become Kamen Rider Dragon Knight. Using his technology, he continues to help rebuild Ventara and continues to assist the Kamen Riders from any more evil that may threaten Ventara and Earth.

Abilities
Eubulon, being the creator of the Kamen Riders, has the ability to manipulate the rider's arsenal. He can change the DNA lock each deck has and reassign it to other people if necessary. He also has the key to the Advent Void, which he created as a fail safe to prevent any Rider from getting killed in action. While the key was damaged with his fight against Xaviax, it still functions well thus making any future vents a non-issue, but as it was damaged, he had to begin repairing it after retrieving only a small handful of Riders. If he ever needs to defend himself, he can use the key to transform into his Rider armor.

The key can also be used as an Advent Deck while in his Rider Form and includes a different card slot format than the others. With his reappearance, the war against Xaviax has changed from a fight for survival to a fight against time. He also creates a new Advent Card called Link Vent and gives it to all 13 Riders. This card, which design looks like Strike's Unite Vent, combines the power of all 13 Final Vents and unleashes it into one condensed attack. This card was specifically made to destroy Xaviax as he had proven to be more powerful than expected.

Advent Deck
Eubulon's slot works very differently than the other Kamen Riders. Unlike the others, his is a swipe-based card reader (located on his right arm) and when swiped, the card burns up (though this may just be due to the fact it was actually Xaviax shape-shifted into the form of Eubulon and copying his powers at the time). In addition, his cards are horizontally designed unlike the other Riders whose cards are designed vertically. Eubulon never actually uses his cards. They are only used in episode 36 by Xaviax in Eubulon's form when he confronts Adam though its assumed that Eubulon has these cards since his Advent Key acts as an Advent Deck.
 * Sword Vent: Conjures a spiked blade like lance that can shoot blue fire.
 * Speed Vent: Accelerates Eubulon to high speeds.

Trivia

 * Eubulon's counterparts in Kamen Rider Ryuki, the Alternatives, are much less heroic figures. Their intentions were good, but they had a ruthless ends-justify-the-means outlook that led them to be considered villains.
 * Footage of both Alternatives is used for the Advent Master form; the two tubes on the shoulders of the suit are black in some shots and silver in others. This being the only outward difference between the two suits, it goes unnoticed by most fans even when clearly visible.
 * Also, while in Ryuki, Alternative and Alternative Zero were man-made riders seperate from the 13, here Eubulon is the creator of the advent decks.