Fans have speculated that he is actually Gerald DeGroot, one of the founders of the Dharma Initiative, even though some say that the writers have debunked that idea.
One thing is certain, we haven't seen the last of him
Holds Kate hostage with what appears to be a German Luger pistol. If this is indeed a German Luger it may be significant as this gun is much older than any other weapon seen on the island so far.
Says that Walt and the others he took including Emma and Zack are still alive
Whilst talking to Jack quotes Alvar Hanso: "Oooo, 50 days. That's what, almost 2 whole months, huh? Tell me, you go over a man's house for the first time, do you take off your shoes? Do you put your feet up on his coffee table? Do you walk in the kitchen, eat food that doesn't belong to you? Open the door to rooms you got no business opening? You know, somebody a whole lot smarter than anybody here once said Since the dawn of our species man's been blessed with curiosity. You know the other one about curiosity don't you, Jack? This is not"