In Exodus Part 2 he was the one who kidnapped Walt (with help of some unidentified people).
Then we met him again in The Hunting Party where he warned Jack, Sawyer, and Locke not to cross "the line" meaning that they were not to explore the northern part of the island. He said that doing so would change the situation from a "misunderstanding" to "something else".
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?"
When Jack accuses him of sending Ethan Rom to infiltrate the survivor's camp and thus learning their names, he enigmatically replies, "Well that's quite the theory."
THEORIES
The nickname Zeke could be a writer included reference to the biblical character Ezekiel.
The nickname Zeke could be because of his stereotypical hillbilly appearance and demeanour.
The nickname Zeke could be may also be a reference to a muppet character with wild hair and a shabby beard.
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.
It is possible Sawyer had previously met Zeke, which would explain why he knew his supposed name
When Jack mentions Ethan Zeke hesitates for a moment, indicating that he might not know Ethan, or that he realized the Losties think Ethan was a member of his group.