From his given nickname, Jack only has one arm and is missing is left arm. He has a scar across the bridge of his nose and always has a stern facial expression.
Jack has a serious personality and when tasked to do something by his superiors, he would follow accordingly. Though his stern nature makes him unfriendly, he is actually a sociable person, as shown when he is with his mercenary group. Jack is also protective of his friends and comrades, even protecting lower members of the Temple Mercenaries while in combat.
But to his opponents such as Carnival, he would taunt them
Even though Jack only has one arm, he is still a competent fighter as he was able to force Carnival into using two swords and clashing with him for a while. He even surprises his opponents and catches them off-guard, due to their underestimation of Jack and his disability.