The Joker is a very rare monster and is one of the only monsters in all games that can only be attained through beating certain battles. The Joker is a legendary monster, albeit being one of the more unknown ones and definitely considered to be a taboo monster. Nonetheless, the Joker is one of the most powerful monsters in the game.
The Joker's appearance earns it its haunting title, looking like something created from a nightmare. The Joker has a giant scythe used as a weapon and has separate parts of his body floating around, akin to the species Durahan, suggesting that it too is the soul of a past human being.