Bartender
The Master thief, Gran.
Sam looked through some of the exploits and was really amused by this person.
Not many people saw him or fought him. Or to put it other way, not many people fought with him and lived to tell the tale.
His battle style is also different and he is the user of the shadow element. At least according to what the five elemental King said.
There are only few assignments that this person failed and one of them is the one with Five elemental King and the other one is the Saber Monarch and both of them are Sam's subordinates.
The Five elemental King's assignment almost succeeded, he managed to grab the item, but he couldn't take it away. The five elemental King locked him in the battle making it impossible for him to escape, even though he disappeared like a phantom.
Five elemental King almost followed him to his hide out. There is nothing he could do and left the item away before escaping.