Joker Undead

Although called the 53rd existence, there are actually multiple Jokers much like in a card deck. If any of them were to win the Battle Fight, it would mark the end of all life on Earth, and the Battle Fight would be reset to begin anew.

Joker
Sometimes referred as the Black Joker, he is true identity of Hajime Aikawa. Technically "wins" the Battle Royale at the end of the series when Giraffa is sealed, but Kenzaki's sacrifice allows Hajime to continue living as a human and need not be sealed, with the Spirit Human keeping him in line. In Missing Ace, he was initially sealed by Blade and remained so in the four-year timeskip. After being unsealed by Leangle to protect Amane from Glaive's assault, Hajime chose to sacrifice himself in order to make Jashin 14 vulnerable for the others to attack, it is unlikely he died however, considering Undead can't be killed. In the series Kenzaki became a Joker to put the Battle Fight at a stalemate, thereby ending it without sealing Hajime.

Joker (Artificial)
Joker reappeared in Kamen Rider Decade in the World of Blade as the final evolution of BOARD CEO Hajime Shijo. Hajime stole the Garren and Leangle Buckles, and used the life forces of Hishigata and Mutsuki to complete the Joker Rouse card, which he used alongside Garren's Buckle to transform himself into the Joker Undead. After assuming the Joker's form, however, he was destroyed by Blade (as a Final Form Ride) and Decade.

Joker (Dai-Shocker)
In Kamen Rider × Super Sentai: Super Hero Taisen, Joker is one of Dai-Shocker's Great Commanders. He takes on the Gokaiger successfully, but is later destroyed by the combined Rider and Super Sentai forces. His human identity isn't made clear in the movie, but an heroic Kamen Rider Chalice is shown as a separate character in the same feature, so, considering his villainous portrayal, he's likely a ressurected Hajime Shijo rather than Hajime Aikawa.

Albino Joker
In Kamen Rider Blade: Missing Ace, another Joker was revealed. This one was white and pink compared to Hajime and is called the Albino Joker by fans. When the fight ended with Kenzaki sealing Hajime, the Albino Joker attacked Karasuma to release the Undead from their Rouse Cards in order to restart the fight. It was then revealed that this Joker is the true identity of Junichi Shimura, who became Kamen Rider Glaive to capture all 4 Category King Undead while targetting the King Categories and Amane Kurihara to awaken Jashin 14. He was defeated due to the sacrifices of Hajime and Kamen Rider Blade King Form and is likely sealed.

Navy Joker
In the finale of Kamen Rider Blade, Kazuma Kenzaki used his King Form to mutate into a Joker, refered to by fans as the Navy Joker in order for Hajime to stay with their friends. His Joker form is never seen but he is seen with a belt with green accents compared to the red ones on Hajime's.