Probably either Fair Fred or Vince. I guess Fred is the most likely choice, since Vince's card with the Seven Dwarfs in the beginning of the game seems to be a challenge to Jack, not a threat.
But that's an EXCELLENT question, actually. It's weird that the guy appears, threatens Jack, then just disappears. Since Jack and Yuki continue their investigations after that, I was expecting the thug to show up again and attack them, then Jack and Yuki could arrest him and interrogate him for information about the Seven Dwarfs. He shouldn't have disappeared without traces (unless Vince killed him and hid the corpse )