Widmore is a rich businessman who was once the leader of the others. After breaking the rules he is banished from the island but spends vast amounts of his resources to try and get back to it. He hires scientists and mercenaries to locate the island and also remove Ben, his replacement from it.
*WARNING - THE FOLLOWING INFORMATION MAY CONTAIN SPOILERS IF YOU ARE NOT CAUGHT UP IN THE SHOW.
Charles Widmore - Bens counterpart, throughout the show it seems that the two of them are at a constant struggle for control of the island (although later on it is revealed that it goes far beyond the two of them). After one of the dharma hatches is destroyed, the island becomes visible to the outside world which it previously was not, Widmore uses this opportunity to send an expedition to the island to remove Ben from it.
Desmond Hume - Desmund is the first person that the castaways meet when they open the first hatch they found. He was shipwrecked on the island from years before during a sailing race around the world and started to work in the hatch with the last living dharma initiative member. He entered the sailing race to try and win back Penelope, Widmore's daughter. Charles did not approve of Desmond seeing his daughter so he thought by winning the race he would earn his respect.
Penelope Widmore - Daughter of Charles Widmore, and longtime girlfriend of Desmund (even though they broke up shortly before he went missing). She continued to look for Desmund the entire time he was missing, somehow knowing that he was still alive somewhere. In season 5 Desmund and her reunite and they have a child together they name charlie, in honor of Charlie Pace from the island.
Matthew Abaddon - One of Widmores employee's, he hires certain members of the expedition team that go to the island on the freighter. He seems to know a lot about the island and even convinced Locke to go on the Oceanic flight 815 knowing that it would crash on the island, although how he knew this is still not clear.
Martin Keamy - Leader of a group of mercenaries that are hired to go to the island and extract Ben. He is especially ruthless and is known to do anything, including killing innocent bystanders to get the job done.
Captain Gault - The captain of the freighter that travels to the island, first name was never mentioned. He eventually thinks Keamy is losing it and taking things too far and confronts him, after a brief dialogue Keamy kills him.
Naomi Dorrit - Naomi is leader of the science expedition which accompanies the group to capture Ben Linus. She is first to arrive on the island and claims that she is their to find Desmund and help all of them get off the island. Locke doesn't believe that she is their to help them and subsequently kills her by throwing a knife in her back.
Daniel Faraday - One of the scientists on Naomi's team, he is their to study the strange laws of physics on the island. He is the son of Charles Widmore although he doesn't know it which makes him the half - brother of Penelope. He is the smartest member on the team, possibly the smartest member on the island and quickly deduces reasons as to why the island is so hard to find. He is in love with Charlotte, another member of the expedition team. His character was named after the 'Faraday Cage' which is a cage where external electric fields have no effect, much like the island isn't effected by outside forces.
Miles Straum - One of the members of Naomi's team, he has the uncanny ability to hear the last thoughts of the dead. Even though he can't hear the thoughts of the living he still seems to be very perceptive to what others are thinking and often makes comments to them about things that he shouldn't know. He later discovers that he is the son of His characters name was chosen because it sounds like Maelstrom, a powerful whirlpool, much like the island which once on is very hard to escape.
Charlotte Staples Lewis - One of the members of Naomi's team, she is an anthropologist. She never knew where she was born but shortly before her death on the island she remembered back to her childhood and revealed that she was born on the island in the dharma initiative. Her characters name was chosen in honor of C.S. Lewis, an author who wrote the book "the lion, the witch and the wardrobe" for which the show has referenced many times and has countless symbols throughout the episodes. Furthermore the island is seen as synonymous with Narnia, both being entire worlds close by but very few people are ever able to find
Frank Lapidus - Frank is a pilot who chauffers groups from the freighter to the island and vice versa. He was originally supposed to fly flight 815 the day it crashed on the island but pulled out at the last second. Part of the reason for volunteering for the expedition was to get some answers about what happened with that plane. His characters name was chosen because of the Frank Lapidus character that has appeared in C.S. Lewis's novels.