Following up on an earlier post, and updating the list of characters. I am sure A) I forgot someone, B) half these people will be dead within the next three weeks, or C) there will be four more new characters by season's end.
Juliet: Scientist Other, on the island seemingly against her will. She's nice to Jack. Sometimes.
Isabel: Grand Inquisitor of the Others' justice system. Or something.
Alex: Ben's daughter. Maybe. Maybe Rousseau's daughter. Who knows? She seems to be in a bit of trouble with the Others.
Karl: Alex's boyfriend. Exiled from the Others.
Nikki and Paulo: These two are in the main cast list. Why? They've done fuck all so far and add nothing to the show. They better become useful real quick. You know, other than muttering out the occasional bit of expository dialogue.
Ben Lucius (AKA Benry): Formerly Henry Gale, balloon boy, now revealed to be leader of the Others.
Tom: Previously known as Cap'n. Beard was a fake. Kind of a 'mo.
Libby: Michael killed her and her interesting back story to save Walt.
Ana Lucia: Killed by Michael too. See above.
Mr. Eko: Killed by the smoke monster for generally being a douchebag in Nigeria. So much for redemption.
(Note, that means all of the Tailies, aside from Bernard, are now dead or assimilated.)
Also dead (Others):
Danny: Killed by Juliet helping Sawyer and Kate escape.
Colleen: Killed by Sun. Married to Danny. I hope they didn't have any kids.
Ethan: He lives on in flashbacks. Apparently being dead doesn't hurt your career as an actor on Lost. See Gunfingers.
Michael: Killed and betrayed his friends to save Walt. Sailed away.
Walt: See above.