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, 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. He took the form of the Navy Joker to put the Battle Fight at a stalemate, thereby ending it without sealing Hajime.