The Joker Undead appeared in 2004 TV series called Kamen Rider Blade. The Joker Undead (ジョーカーアンデッド, Jōkā Andeddo), usually referred to as simply Joker (ジョーカー, Jōkā) is a special Undead that exists outside the standard four suits and is not the ancestor of any living being.
The Joker is one of the strongest Undead and is the only one that can seal other Undead into Rouse Cards without the Monolith. By scanning a Rouse Card with the Rouzer on his abdomen, it can transform into the corresponding Undead. Joker's ability to seal Undead and use Rouse Cards forms the basis for BOARD's Rider System.
If the Joker wins the Battle Fight by being the last Undead remaining, Darkroachi will endlessly emerge from its shadow and the Monolith in order to reset the Battle Fight by killing all living things, regardless of his wishes. As the winner of the Battle Fight, iy will be granted the power to shape the world like other Undead. Evidently, the rules of the Battle Fight do not only apply to the real world, as the as the Game World has also been destroyed due to the Joker being the only Undead present.
Hajime Aikawa is the original Joker, the 53rd participant of the Battle Fight. Once unsealed in modern times, he is manipulated by the Human Undead into assuming a human identity, gaining a sense of empathy in the process. Having become a member of human society, Hajime uses the powers of the Mantis Undead to masquerade as Kamen Rider Chalice.
In the finale of the series, Hajime becomes the last Undead standing, which awakens his instincts as the Joker. To save Hajime and the world itself, Kazuma Kenzaki intentionally overuses King Form and mutates into a second Joker. Responding to this, the Monolith stops the world's destruction, before ordering Hajime and Kazuma to fight each other. Kazuma adamantly refuses to do this, outright disobeying the Monolith's commands. Knowing that his presence could potentially cause Hajime to lose control once more, Kazuma then willingly disappears from his friends, though not before vowing to one day overcome his fate as an Undead.
In Kamen Rider Zi-O, fourteen years later since Kazuma's mutation as an Undead, Amane Kurihara is forced to transform into Another Blade by White Woz. To save Amane, Hajime transforms into Kamen Rider Chalice once again, resulting in Kazuma losing control over his Undead instincts. Ultimately, the two have their Undead powers absorbed by Another Blade, disqualifying them from the Battle Fight. After Sougo Tokiwa has resolved the situation, the Battle Fight effectively ends, with the Monolith itself vanishing, no longer able to sense the presence of any Undead.
Around 6 years afterwards, the Destroyer turns Hajime and Kenzaki back into Joker Undeads so as to start the Battle Fight anew and sow chaos within the world. Although the heroes manage to foil the scheme of the Destroyer, Hajime and Kenzaki still remain Jokers. As such, the latter have to leave humanity once again to avoid further conflict with the former. Despite this, Sakuya Tachibana vows to some day restore Kenzaki's humanity.
Kamen Rider Blade: Missing Ace explores a different continuity than the events of the series' finale. In this timeline, Kazuma chose to seal Hajime away, but it inadvertently unsealed the Albino Joker (アルビノジョーカー, Arubino Jōkā). Being designed to be a back-up Undead that could start the Battle Fight once more, the Albino Joker unsealed several Undead that were sealed away, killing Kei Karasuma in the process.
As the 54th participant in the third Battle Fight, it assumes the identity of Junichi Shimura and infiltrates BOARD as Kamen Rider Glaive, later murdering Shin Magaki and Natsumi Miwa for their Category King cards to summon Jashin 14.
In the fight against the Albino Joker, Kazuma orders Leangle to release Hajime from his card, reuniting the four Kamen Riders. When Amane is used as a sacrifice for Jashin 14, Hajime offers himself up instead, allowing Blade King Form to destroy Jashin 14 and the Albino Joker for good.
In the World of Blade as depicted in Kamen Rider Decade, the Battle Fight does not appear to exist, with the existence of the Joker Undead being unclear. Kamata had worked with the CEO of BOARD Corporation Hajime Shijyo to create an artificial Undead using a combination of DNA taken from the Category-Ace Undeads used by the Rider System (Beetle, Stag, and Spider), and the life-forces of both Sakuya Hishigata and Mutsuki Kuroha. By scanning the completed Joker Rouse Card into the Chalice Rouzer, Hajime can transform into the Joker Undead. He is later destroyed by Kazuma and Tsukasa.
In Kamen Rider × Super Sentai: Super Hero Taisen, Joker is one of the monsters ressurected by Doctor G and becomes one of Dai-Shocker's Great Commanders. He takes on the Gokaiger successfully alongside other commanders. In the final battle, Joker is seen for the last time facing Red1.