Jack (Matthew Fox): A doctor and the de facto leader of the castaways, Jack is gruff and sometimes a little self-important, but he's also good at getting things done and holding the group together in the face of a crisis.
Kate (Evangeline Lilly): The meat in the love triangle sandwich between Jack and Sawyer (which producers promise will be resolved early this season). Also a fugitive from justice who was in handcuffs when the plane went down.
Sawyer (Josh Holloway): Con man and lover of nicknames. Alternates between resourceful ally and cunning adversary for the rest of the castaways. Frequently shirtless.
Hurley (Jorge Garcia): Loveable fat guy who gained some depth in Season 2. Hurley won the lottery by playing the cursed numbers (a big part of the show's mythology), spent time in a mental institution and made a connection with the now-dead Libby that has yet to be fully explained.
Locke (Terry O'Quinn): Former life-long loser reborn as a wisdom-dispensing sage on the island. Also former paraplegic granted the use of his legs by the island's mysterious forces, which may or may not have the power to heal people.
Jin (Daniel Dae Kim) and Sun (Yunjin Kim): Korean married couple whose past includes working for a crime lord (him) and secretly learning English (her). Past misunderstandings aside, they're now very much getting along, and Sun is pregnant despite Jin's previous impotence, yet another possible healing power of the island.
Eko (Adewale Akinnuoye-Agbaje): One of the "Tailies," passengers from the tail section of the plane who were thought dead after the crash. Since joining the rest of the castaways, he's made himself known as a spiritual leader and a fierce warrior.
Sayid (Naveen Andrews): The former Iraqi soldier has proved resourceful in battling island antagonists and enjoyed a short but doomed romance with blond, pampered Shannon.
Claire (Emilie De Ravin): Pregnant for much of the first season, Claire finally gave birth to baby Aaron, who is either some kind of messiah or just a baby with an abnormally large head.
Charlie (Dominic Monaghan): One-time minor rock star (check out his "band," Driveshaft, at www.driveshaftband.com) and heroin addict, now mooning after Claire when not having disturbing visions about protecting her baby from evil forces.
Henry Gale (Michael Emerson) and The Others: No, not a rock band, but rather the chief threats to our intrepid heroes. Who are the Others? Where did they come from? What do they want? Very few answers have been forthcoming in the show's first two seasons, although the captured Henry Gale, who at first claimed to be a fellow castaway, offered some insight when he chillingly claimed in the second-season finale that he and his baby-snatching, Tailie-murdering gang were actually "the good guys."
Michael (Harold Perrineau): Busy for much of Season 2 searching for kidnapped son Walt, Michael struck a deal with the Others for the return of his son that involved betraying the rest of the castaways. The Others seemed to honor their deal, and Michael was last seen speeding away from the island on a boat with Walt.
Walt (Malcolm David Kelley): Son of Michael and subject of much fascination for the Others. May or may not have psychic powers of some kind. Gone for now, but likely not forgotten.
The Hatch: The central focus of Season 2, this underground bunker is the abandoned research facility known as Swan Station, part of a network of such outposts throughout the island. While the true purpose of these hatches remains a mystery, signs indicate that they are part of some grand psychological and perhaps metaphysical experiment. Inside Swan Station was a computer that required Hurley's cursed numbers to be input every 108 minutes or ... something bad would happen. In the second-season finale, that something bad finally happened, and the fallout should dominate the new season. (As for what actually happened, the overriding theory is that some sort of electromagnetic pulse was released, which was also what caused the plane to crash initially.)
The Hanso Foundation: Shadowy organization behind the research projects on the island, represented by grainy "orientation" films discovered by the castaways. Likely behind the entire, vague conspiracy, given that flashbacks have shown several nefarious characters to have connections to the group. Also, not quite shadowy enough not to have its own website (www.thehansofoundation.org). Shannon (Maggie Grace): Dead, shot accidentally by Ana-Lucia.
Ana-Lucia (Michelle Rodriguez): Dead, shot not so accidentally by Michael.
Libby (Cynthia Watros): Dead, shot by Michael.
Boone (Ian Somerhalder): Dead, but set to appear in a flashback in Episode 2 of the new season.
Desmond (Henry Ian Cusick): Previously shipwrecked on the island while attempting to sail around the world. Discovered in the Hatch at the beginning of Season 2, Desmond ran off, leaving the castaways to input the numbers into the computer. He returned in the season finale after failing to escape the island, and last year's big cliffhanger saw Desmond's long-lost love out in the real world ... discovering where the island is located.
Vincent: Walt's dog. Did not get to escape with Walt. Enjoys barking.