Although called the 53rd existence, there are actually multiple Undeads known as Joker Undead or simply called the 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.
Sometimes referred as the Black Joker, he is the 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. However, this causes both Kenzaki and Hajime to not meet each other again. Since, they will be forced to fight until dead, as they are now rivals.
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 as Chalice 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.
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 seemingly defeated by the combined Rider and Super Sentai forces. He doesn't make many appearences during the war, although when the Gokaigers and Kamen Rider OOO are being attacked by Rider Hunter Silva, he is seen fighting Red1 of Bioman, who is encouraging Gokai Silver to show Silva the power of a real silver. His human identity isn't made clear in the movie.