Kamen Rider Onyx is a copy of Kamen Rider Dragon Knight armed with a black Japanese Dragon Advent Deck. His Rider appearance resembles Dragon Knight, but with a black look ripped all over and is similar to Dragon Knight's Blank Form. Onyx is a newly created Kamen Rider, which is why no other Rider recongnized the armor, thus increasing the total number of Kamen Riders to 13. He first appeared to ambush Kit Taylor and knock him into Ventara. When Kit is taken down by Onyx, it turns out to be a nightmare. The next time Onyx shows up is actually Kit (as Dragon Knight) retransforming into the black-armored Onyx venting Wing Knight and confronting Siren. However, it turns out to be just another nightmare.
Onyx originally represented the struggle that Kit was going through, confronting Onyx when he was feeling useless because of his jealousy towards Kase and using Onyx's power when confronted by Xaviax about his father. This was due to a phenomenon that Chance and Price refer to as the Advent Beast's rage imposing its will on the rider's mind.
When Kit is taken out of the Advent Void, Eubulon tries to give Kit the Onyx Advent Deck. Kit initially refuses to use it due to his nightmares, but accepts it when Eubulon and Kase reassure him that those events were only nightmares. Kit uses its power to help Len and the other Riders. He later confronts Adam, who has the Dragon Knight deck, and fights him until Eubulon interrupts the battle. Kit joined forces with the Ventaran Riders, and worked with them to defeat Xaviax, first by defending Earth from attacking Mirror Monsters and also working to render Xaviax's teleporters useless via the virus used to negate them. Kit is present when the team travels en masse into Ventara to storm Xaviax's headquarters, and also joins the other twelve Riders in using the Link Vent to destroy the alien tyrant.
Kit retains the Onyx armor until the final episode of the series, when he reclaims the Dragon Knight deck/armor when Eubulon gives him the choice of which deck to use. The current whereabouts and user of the Onyx deck, if one exists, are unknown.
Being an exact copy of Dragon Knight, Onyx is a "balanced" Rider with equally powerful melee and far-range fighting capabilities, with a variety of cards at his disposal using his own dragon to help fight. Because Onyx is exactly the same as the Dragon Knight armor, Kit has no trouble using it. The Onyx deck is unique amongst the Advent Decks as the dragon emblem on the deck is black rather than gold.
Visor and Advent Deck
Like Dragon Knight, Onyx slots his cards on the dragon-like slot on his left arm called the Black Drag Visor.
- Attack Vent (AP: 6000): Summons Dragblacker to directly attack his opponents. Dragblacker is essentially a black version of Dragredder.
- Sword Vent (AP:3000): Conjures a black dragon-designed broadsword, modeled after Dragblacker's tail.
- Strike Vent (AP: 2000): Attaches a black dragon-like gauntlet on his right hand to shoot a burst of black flames assisted by Dragblacker.
- Guard Vent (AP: 3000): Attaches black dragon-like guards on his arms.
- Final Vent (AP: 7000): Summons Dragblacker for Onyx'sFinal Vent. It consists of Onyx jumping into the air to emit dark energy that paralyzes the opponent's legs and then deliver a flying side kick with Dragblacker flying behind to boost the attack's power with its dark flame.