Created by the "Advent Master" Eubulon, the original 12 Kamen Riders were the chosen defenders of the "mirror world" known as Ventara. With the exception of Wing Knight (and later Siren), they were all killed (referred to as being "vented") in battle with Xaviax when one of their own (the original Dragon Knight) betrayed them and their Advent Decks were stolen. Since the Decks were designed to work with only one person, each deck only functions again when given to humans on Earth that are "mirror twins" of their Ventaran counterparts, with matching DNA or when Eubulon reassigns the deck. To access the powers, the chosen deck wielder holds the Advent Deck carrier in front of them to activate a burst of energy that creates the Advent Belt holder for the deck while calling out the words "Kamen Rider". The deck is then inserted in the belt and spins, further activating a pair of rings that surround the bearer and creates the armor for the specific Kamen Rider. The energy emitted from those rings can be used to repel others away while transforming. Each Kamen Rider has a special sense that can detect Mirror Monsters and other Kamen Riders . The decks also give the Riders the ability to travel from one place to another instantly by traveling through any mirror or reflective surface and traversing the mirror world to end at their intended (or at times unintended) destination(s), be it in their Rider armor or human form. They can also travel with other humans or push them through a reflective surface to the other world as long as they keep in contact with them while traveling. If either side of the mirror is blocked or destroyed, they cannot travel through that mirror. Riders usually travel through the dimensions with a specialized motorcycle parked in between the dimensions called an Advent Cycle. A Rider may also travel with their own motorcycle which, upon transforming, will change the motorcycle's appearance as well.
The Kamen Riders will be assisted by their own Advent Beast. Once a Mirror Monster is confined via their contract cards, it forms a Contract with the Rider who captured it. This changes the Rider's armor based on the Advent Beast confined. The Advent Beasts support their human partners through the Advent Decks, usually appearing when an Attack Vent or a Final Vent is used. Should the Advent Deck be destroyed or damaged in combat, the Advent Beast and its owner will disappear to the Advent Void. The Advent Beast can only return if the contract card is reactivated. Because of the nature of the contract, once it is established, a Rider cannot simply quit being a Kamen Rider because as Len describes it, it constantly calls Riders back to fight monsters and protect the people.
The Riders use their powers (via the advent cards) to battle either the Mirror Monsters or each other. Each card can be slotted into a device the Rider wears to access a "Vent" attack or ability such as Sword Vent, Strike Vent or Copy Vent. Most equipment based cards are based on that Rider's respective Advent Beast for example Dragon Knight's Sword Vent is based on Dragredder's tail. Each Rider has his own deck designed for that Rider only, even if a Rider slots a card belonging to another Rider, it still goes to the card's owner rather than the one who slotted it. Each Rider possesses a card to summon their respective Contract Monster (Attack Vent) and another card for their Final Vent, a devastating attack that involves using the Advent Beast's power to finish off their opponent. Depending on the power of the attack and the strength of the target, the opponent may either be just defeated, or the attack can possibly "vent" them. A vented Kamen Rider and Advent Beast are sent directly to the Advent Void, a world between the mirrors where they are trapped and are never seen again, leaving only their Advent Deck behind. The Advent Void was designed as a fail safe in case a Rider has been badly damaged leaving them there to recover until Eubulon could retrieve them, this allows them to fight without having to worry about being taken out phycially. However, since Eubulon was inactive, the Advent Void ended up trapping Riders rather then simply saving them thus making the war thus far a fight for survival.
In addition to their normal cards in their Advent Deck, one more powerful card can be used called Survive. This allows the Riders to use the powerful Survive Mode. With it, they acquire new armor, new Visor Slots and new cards to accomidate the new form. However, it uses too much energy and therefore, a Rider cannot access the Survive Mode for too long. As a result, it is used in emergencies and taken out of the deck beforehand. Kase only manages to obtain the Survive cards for Dragon Knight and Wing Knight before initially escaping. Thus Dragon Knight and Wing Knight are the only Riders who can use Survive Mode.