Although debuting over a decade ago in 2004, Lost remains an enduring classic of dramatic television. During its run, fans composed elaborate theories to explain the mysterious events that took place from episode to episode, the mystique of series' supernatural elements as well as the complex and labyrinthine plot. It was a phenomenon which garnered both critical and commercial acclaim.

But one of the pillars of Lost's lasting appeal is the character-driven story arcs and episodes spotlighting individual survivors on the enigmatic island somewhere in the Pacific. Clocking in at over one hundred episodes over the course of six seasons, keeping track of the characters—their mannerisms, as well as their inspirations and motivations—can be daunting. And while the series boasts a bevy of characters great and small, this list should serve as a retrospective—or “crash course,” if you'll excuse the pun—for the most significant pieces on this grand chess board of television history.

Warning: For those you who haven't seen the series, SPOILERS follow.

22. Kate

Portrayed by: Evangeline Lilly

First Appearance: Pilot (Season 1, Episode 1)

Kate's past haunts her present; her survival of Oceanic 815 crash gives her a chance to evade her past life as a fugitive of the law for murder. But Kate rarely comes across as a hardened criminal; that said, she is strong-willed and determined, capable and smart, her survival skills having been honed on the run. She later is one of the few survivors to escape from the island, and the only one to do so twice. She is part of a love triangle with Jack and Sawyer, drawn to Jack's dependability and Sawyer's wild side. Although Kate's past should color the perspective of other survivors, she is often regarded as a voice of reason and dependability.

Trivia: Originally, Jack was intended to die during the pilot, and Kate would have been cast in the “leader” role for the survivors.

21. Hurley

Portrayed by: Jorge Garcia

First Appearance: Pilot (Season 1, Episode 1)

Hugo “Hurley” Reyes is arguably the warmest, most beloved character in Lost. Possessed of a kind demeanor and general acceptance of others, he nonetheless harbors a cursed past... at least to Hurley. Prior to the crash, Hurley was both in an asylum patient and a lottery winner, both incidents tied to a series of fated numbers which reemerge for Hurley on the island, a sign that all of the coincidences are tied together in a lattice of fate controlled by some other force. It is no coincidence then, that Jacob later affords Hurley special knowledge about his grand plan. Oh, and Hurley can see and communicate with ghosts.

Trivia: Season 2 introduced a love interest for Hurley in the form of Libby (Cynthia Watros), a clinical psychologist who also happened to attend the same mental institution Hurley attended.

20. Charlie

Portrayed by: Dominic Monaghan

First Appearance: Pilot (Season 1, Episode 1)

Bass guitarist for a rock band called Drive Shaft, Charlie Pace is one of the first characters explored in Lost. Revealed to be a heroin addict early on, some of his biggest struggles in season 1 involve him concealing his addiction from the other survivors, something which Locke ultimately helps him overcome. Charlie falls in love with Claire, who returns the affection, and he takes the role of a father for her newborn son, Aaron, while they are on the island. His fate is twice foreseen by Desmond, the second instance showing his ultimate sacrifice on the Dharma station called “The Looking Glass.”

Trivia: Charlie's rock and roll background comes in handy when a musical combination to disable a jamming device requires him to recall The Beach Boys' “Good Vibrations.”

19. Desmond

Portrayed by: Henry Ian Cusick

First Appearance: Man of Science, Man of Faith (Season 2, Episode 1)

First revealed to be the “man in the hatch” discovered by Jack and Locke during season 1, he has been consigned to enter a series of numbers into a computer every 108 minutes to “prevent the end of the world;” and his adventures only get more interesting from here. Desmond is the equivalent of an “X Factor” for the story of Lost, a karmic wild card who often finds himself at the center of everything from time travel, the first individual capable of discerning the true nature of the “flash-sideways” in season 6, and being acquainted with people like Charles Widmore (Alan Dale), whose involvement in the Dharma Initiative directly influences events on the island. When Desmond is involved, big things happen.

Trivia: Desmond's comment to Jack shortly after they first meet, “see you in another life, brother,” takes on a whole new level of irony in season 6.

18. Mr. Eko

Portrayed by: Adewale Akinnuoye-Agbaje

First Appearance: Adrift (Season 2, Episode 2)

A member of the “tailies,” the name given to the survivors of the Oceanic 815 crash from the tail section, Mr. Eko is one the most mysterious characters of Lost, prior to his flashback episode, “The 23rd Psalm.” A man of few words—fewer following the tailies ambush by the Others, and Mr. Eko's killing of two of them, where he forces a penance of silence upon himself—Mr. Eko has a faith he has reclaimed following his dark past in Nigeria as a drug lord. His guilt spurs him into posing as the priest his deceased brother Yemi (Adetokumboh M'Cormack) was, who tried to turn him away from his wicked ways. Mr. Eko's faith in the mysteries of the island sometimes puts him at odds with characters like Locke, but motivates him to build a church on the island with Charlie's help.

Trivia: The Beechcraft aircraft in the jungle, first appearing in “Deus Ex Machina,” was the plane Mr. Eko's brother was on when it crashed, upon which Mr. Eko was originally intended to be a enger.

17. Richard Alpert

Portrayed by: Nestor Carbonell

First Appearance: Not in Portland (Season 3, Episode 7)

Among the already interesting and diverse characters in Lost, Richard is one of the most unusual by virtue of his gifts and his origin. Hailing from the Canary Islands in the 19th century, Ricardo (as he was called) was a man convicted of the murder of a doctor, sentenced to death, sold into slavery, and then shipwrecked on the island. He is one of the earliest examples of a pawn in the battle of good and evil between Jacob and the Man in Black, and as such was gifted with immortality. He remains one of Lost's greatest mysteries, since he always appears the same age between flashbacks and timelines as far back as 1954, a detail which should rightly unsettle viewers.

Trivia: Nestor Carbonell is fluent in Spanish, a skill which he was able to put to use in Richard's origin episode, “Ab Aeterno.”

16. Michael and Walt

Portrayed by: Harold Perrineau (Michael) and Malcolm David Kelley (Walt)

First Appearance: Pilot (Season 1, Episode 1)

Father and son, Michael and Walt both take part in some of the bigger twists in the already complex story of Lost. Michael's obsessive protectiveness of Walt makes him appear as domineering or shallow to his son's bigger needs. For Michael, finally gaining custody after the ing of Walt's mother means he must protect Walt at any cost, and his willingness to achieve this is foremost in his mind; everything else is expendable, a mentality tragically exploited by the Others. Walt is a bigger mystery, originally portrayed as being in tune with the supernatural elements of the island, and theorized to be one of the potential “candidates” intended by Jacob to inherit the island. Walt loves his dog, Vincent, who is featured in bookending shots in the first and last episodes of Lost.

Trivia: Michael and Walt are the first survivors to leave the island, effectively written out of the majority of the series for the most part. Popular opinion points to this being a result of Malcolm David Kelley growing old too quickly for the show, and was no longer convincing as a ten-year-old.

15. Daniel Faraday

Portrayed by: Jeremy Davies

First Appearance: The Beginning of the End (Season 4, Episode 1)

First introduced following with the freighter presumed to rescue the survivors, Daniel Faraday is a theoretical physicist with a focus in space-time. At first glance, he comes across as distracted, but who wouldn't be distracted when surrounded by existential paradoxes and the various manifestations of time travel? Faraday's lineage is tied inexorably to the island, his parents Eloise Hawking (Fionnula Flanagan) and Charles Widmore being the effective progenitors of the Dharma Initiative.

Faraday's presence and knowledge proves crucial to Desmond's survival following his personality flipping between time in “The Constant,” and his understanding of time and destiny makes him a scientific counterpart to Desmond in a way. His affection for fellow member of the freighter, Charlotte (Rebecca Mader), creates a closed loop of consequence, where he tries to dissuade her as a child from returning to the island, invariably propelling her on that path, reinforcing Daniel's assertion that time cannot be altered.

Trivia: The mouse which Faraday uses in order to further his studies of time-displacement is named “Eloise” after his mother.

14. Sun

Portrayed by: Yunjin Kim

First Appearance: Pilot (Season 1, Episode 1)

Sun-Hwa Kwon was always meant to be in a position of power and influence, but is introduced into Lost as a woman forced to endure the nigh-oppressive control of her husband, Jin. In Korea, Sun was born into the powerful Paik family, and was surrounded by wealth. Her marriage to Jin was troubled, and led to an affair with a former suitor and her English language coach, Jae Lee (Tony Lee), creating a double secret from her husband revealed over time on the island.

Sun escaped the island as one of the “Oceanic Six,” and assumed control of her father's company. This power afforded her some influence to directly confront those whose involvement in the affairs of the island, including Ben Linus, whom she believed was in part responsible for Jin's fate on the freighter. Sun may be unassuming at first, but she can wield an icy rage against those who cross her.

Trivia: Actress Yunjin Kim originally auditioned for the role of Kate; while that role went to Evangeline Lilly, the producers created the role of Sun specifically for her.

13. Jin

Portrayed by: Daniel Dae Kim

First Appearance: Pilot (Season 1, Episode 1)

Jin-Soo Kwon is the husband to Sun, who, unlike Sun, was not born into power and wealth, but instead into poverty, the child of a fisherman and a prostitute. His lineage a mark of shame following his courtship of Sun, he becomes an enforcer pressed into illegal and violent activity on behalf of Sun's father. Jin's path into darkness is largely unknown to Sun, but its aftereffects color their relationship, leading to their strained marriage when we are first introduced to them.

Jin is left behind on the freighter in season 4's finale, leading Sun (and audiences) to believe him dead. However, he survives and ends up bouncing through time to encounter a young Danielle Rousseau (Melissa Farman) in 1988, and ing other survivors during the formative years of the Dharma Initiative in 1974.

Trivia: Originally the only apparent survivor of the Oceanic 815 crash who does not speak English, Jin eventually learns the language during his three years with the Dharma Initiative.