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Joker was the term used by the Black Serpent Mafia to call their agents who specialized in the 'versatility' course. And these guys were the ones who were said to be the best agents of the Mafia.
Their training regime made them the jack-of-all-trades which led them to have a broad pool of knowledge, while having a vast number of aces under their sleeves.
They were the type of agents who were fighting with a lot of reserves. They could break their opponents' mentally during battle by slowly showing their cards one by one. It was as if those opponents were being thrown in an illusion where all of their moves were being overwhelmed brutally.
Even though these agents were considered to be the most skilled agents of the Mafia group, encountering an agent like them was quite rare. After all, training a person with a vast set of skills was quite hard, much less someone who was skilled in different fields.