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Jennifer Connelly as Melanie Cavill: The powerful Head of Hospitality (the department responsible for smooth relations) and the Voice of the Train (responsible for making the PA system's daily announcements) aboard Snowpiercer. To the majority of the train, Melanie is believed to be Mr. Wilford's representative relaying his orders to the passengers and crew. It is later revealed that Wilford was not initially aboard the Snowpiercer. Melanie has been facing the crushing burden of directing Snowpiercer on its global journey, managing mounting resource and class issues, having her authority and decisions challenged, and maintaining Wilford's iron order throughout the train to preserve the last of humanity. She desperately tries to maintain the myth of Wilford, which is known to only a few other passengers. Ultimately the secret is exposed, leading to a coup attempt by wealthy passengers from First Class. This coincides with a revolution by the Tail and Third Class sections, in which Melanie helps out in the end and saves it from certain defeat. After the revolution, Melanie officially hands leadership of the train to Layton and resumes her duties as Head Engineer, but her plans to remain indefinitely in the engine room are threatened by the arrival of Mr. Wilford and the surprise return of her believed dead daughter Alexandra. In the second season, Melanie departs Snowpiercer to find a place that humanity can recolonize. In the second-season finale, Layton and Alex discover that Melanie had sacrificed the last of her power to protect her data before seemingly walking into the Freeze to die. In season three, Melanie appears as a hallucination to both Alex and Wilford. It's eventually discovered that she had survived by taking a track scaler from a nearby hangar and using the suspension drug to keep herself in hibernation. After being rescued, she betrays Layton and seizes control of the Engine before agreeing to split the train, with Melanie leading those who want to stay onboard Snowpiercer.
Daveed Diggs as Andre Layton: A former detective turned leader of the Tail, a group of passengers who boarded Snowpiercer by force without tickets and are treated poorly as a result, often having their food, water, and electricity cut. As the world's only surviving homicide detective, Layton is reluctantly deputized by Melanie as Snowpiercer's Train Detective to help solve a series of murders. While investigating the mysterious killings, he gathers intelligence and allies to support a revolution by the Tail and Third Class sections. After First Class has overthrown Melanie, they start to crush Layton's revolution in a series of bloody confrontations. It is saved only after Melanie teams up with Layton in the end. After the revolution's success, she officially hands the leadership of the train to Layton, but his plans to implement a democratic government are threatened by the arrival of Mr. Wilford. At the end of the second season, Layton creates his own pirate train out of the first ten cars of Snowpiercer to resist Wilford, taking with him several of his and Melanie's most trusted loyalists. At the end of the third season, Layton takes Big Alice and several people to a habitable valley in the Horn of Africa. In season four, Layton has stepped down as the leader of humanity in favor of a democratically elected government, although he still serves as an elected member of the new town council, stating that he's a wartime general, not a peacetime leader. This allows Layton to enjoy a simpler life before the murder of Zarah and the kidnapping of Liana causes Layton to step back up as the leader of humanity against a threat that endangers the remainder of life on the planet. At the end of the series, Ruth decides to share leadership of New Eden with Layton as co-chairs of the council.
Mickey Sumner as Bess Till: A Brakeman and part of the train's security force, Till was a rookie officer in the Detroit Police Department and witnessed its self-destruction during the Freeze. She is in a romantic relationship with Jinju and later upgrades from Third to Second Class to be with her. Tormented by the injustices of the class system, however, she ultimately decides to support Layton's revolution. Because Till is afraid to share what she knows about Layton's moves, while Jinju still wants to keep her knowledge about First Class and Melanie secret, they eventually break up. After Josie, one of Layton's allies, finds him in medically induced suspension, Till helps Josie and Layton escape. She also tries but fails to save Josie from interrogation by Melanie. In season 2, Layton promotes Till to Train Detective to solve a series of crimes involving members of the Tail and the Breachmen. It is later revealed that Mr. Wilford was behind them as a way to undermine Layton's credibility and leadership. Till leaves with Layton's pirate train at the end of season 2. After the trains reconnect in season 3, she resumes her role as Train Detective, investigating Pike's attempts to assassinate Layton and forming a romantic relationship with Miss Audrey, who has been Mr. Wilford's love interest. After Big Alice separates from Snowpiercer to embark on its journey to New Eden, Till becomes the new Lead Brakemen aboard Snowpiercer.
Alison Wright as Ruth Wardell: Melanie's deputy in Hospitality who looks after First Class passengers and delivers demands and punishments to the Tail. Before the Freeze, she ran a bed and breakfast in Kendal and was personally recruited by Mr. Wilford when he stayed there as a guest. A friend to Melanie, Ruth feels betrayed after the secret about Mr. Wilford is exposed. She becomes Head of Hospitality and is initially thrilled by Mr. Wilford's return. When questioned by Layton, she insists that her loyalty is to Snowpiercer and its passengers. She subsequently joins Layton's leadership council with Miss Audrey, Roche, Bennett Knox, and others but he is reluctant to trust her. Ruth is inadvertently left behind with Wilford after Layton creates a separate pirate train. In season 3, she becomes the leader of a resistance movement against Wilford on Snowpiercer before resuming her duties as Head of Hospitality after Layton's return. She later chooses to join him in going to New Eden where Ruth becomes the town's mayor and the chairwoman of the town council after Layton decides to step down as the leader in favor of a democratically elected government. In the series finale, Ruth decides to make Layton her council co-chair rather than continuing to lead it on her own.
Lena Hall as Miss Audrey (seasons 1–3; recurring season 4): Chanteuse and the lead madam of Snowpiercer's Nightcar, a Third Class den of (mostly) platonic prostitution and spiritual healing. She represents Third Class before the rest of the train and is very vocal about addressing the social differences on Snowpiercer and supports Layton's revolution. In season 2, she struggles with the return of Mr. Wilford; she was once an escort who served Wilford personally for years and still feels drawn to him despite fearing that renewing their relationship would make her life worse. It is later revealed that the two have a sadomasochistic relationship with Miss Audrey being the dominant party. She later defects to Wilford's side, helping him against rival factions. When Layton creates a separate pirate train, she is taken hostage by his crew to ensure the safety of Zarah, who was pregnant and stayed behind on Wilford's part of the train. After Snowpiercer was reconnected, Audrey is exiled to Third Class and banned from the Nightcar. She eventually forms a romantic relationship with Till who helps her move past Wilford and begin helping people again.
Iddo Goldberg as Bennett Knox: One of Snowpiercer's engineers, he has inside knowledge of the initial absence of Mr. Wilford and knows the deepest secrets of the train. He is fiercely loyal to Melanie, with whom he is romantically involved. When Mr. Wilford returns, he betrays Melanie as he believes that Snowpiercer would not survive without the supplies onboard Wilford's Big Alice. At the same time, he allies with Layton to protect the train together in Melanie's absence. He subsequently acts as part of Layton's leadership council. Bennett was in on Melanie's plot to steal Snowpiercer in the first place unlike his colleague Javi, another train engineer, and stresses that they had to depart the station before Wilford boards the train and more people get killed, even though Melanie's daughter had not arrived yet and would miss the train if it leaves. In season two, Bennett joins Layton's pirate train. At the end of season three, Bennett chooses to remain on Snowpiercer with Melanie rather than going to New Eden. In "The Engineer", Bennett dies when he sacrifices himself to separate Snowpiercer and Big Alice via manual override, freezing to death in the process. Subsequently, the passengers carve an inscription over the Big Alice helm to honor Bennett and his sacrifice. Bennett's friends are shown to take his sacrifice very hard.
Susan Park as Jinju Seong (season 1): Snowpiercer's agricultural officer from Second Class and a member of the train's elite. She has been in a romantic relationship with Bess and has inside knowledge of Mr. Wilford's initial absence. After the revolution, she and Bess break up.
Katie McGuinness as Josie Wellstead: A strong, no-nonsense Tailie, she cares for Miles and other children in the tail, and uses training from her time as a veterinarian before the Freeze to treat the sick or injured. She is Andre's love interest after Zarah departs from the tail. She has been part of the uprisings in the tail and takes great risks on behalf of her community. She is later thought to have been frozen to death after trying to kill Melanie who tortured her. In the second season, it is revealed that she has survived, albeit severely injured. Receiving treatment from the Headwoods, Wilford's scientists on Big Alice, Josie becomes resistant to extremely cold temperatures like Icy Bob and receives a new prosthetic hand. She later uses her new ability to help Layton escape and joins him and others on the pirate train. In the third season, Josie has a brief fling with Bennett Knox. When the time comes to reconnect the pirate train with the rest of Snowpiercer and Big Alice, she uses her new ability to help Layton again. She later decides to join others in going to New Eden.
Sam Otto as John "Oz" Osweiller: A young Brakeman and Bess's patrol partner as part of the train's security team. He is less scrupulous than Bess in dealing with the passengers. Oz was raised by a single mother who was a prostitute and emotionally distant from him. He references being in pain from "everything", which is also his rationale for taking Kronole, a street drug likely derived from the medication used for the suspension of life. During Layton's revolution, Oz abandons his job with the Brakemen. He begins a relationship with LJ and becomes the head of janitorial after the assassination of Terence. In season three, during Audrey's absence, he and LJ run the Night Car together as a bar and eventually get married. However, LJ remains deeply committed to her own machinations to rise to the top again, one time even threatening Oz when he appears reluctant to support her. Oz later leaves LJ to join the colonization of New Eden. After initially becoming a recluse and a hermit, Oz accepts an offer from Roche to serve as a security officer for New Eden, taking over for Roche after he mysteriously vanishes during a blizzard. Oz eventually discovers that the mysterious voice he keeps hearing isn't LJ's ghost but rather Buffalo, the Animal Squad Engineer, who was spying on the town.
Sheila Vand as Zarah Fahrami (seasons 1–3; guest season 4): Layton's ex-wife who, unable to adjust to the poverty of the Tail, left to become a bartender in the Nightcar, antagonizing other Tailies and breaking Layton's heart in the process. She later becomes pregnant with Layton's child. After the revolution, she tries to build a new life with him. In the second season, she joins Hospitality to bridge the gap between Layton and Ruth Wardell. When Layton creates a separate pirate train, Zarah decides to remain behind, feeling that she is safer there and that Wilford would not dare to hurt her due to her pregnancy. She allows Wilford's scientists to perform an experimental procedure to augment her baby with resistance to extremely low temperatures. In season three, she gives birth to Liana Layton. In the season 4 premiere, she is thrown off of a cliff to her death by Rat, one of Milius' men, when he kidnaps Liana with the help of Dr. Headwood. However, both Headwood and Milius claim that Zarah's death was unintentional. Her death is later avenged when Layton kills Rat in the series finale with Boki's help.
Roberto Urbina as Javier "Javi" de la Torre: One of Snowpiercer's engineers, he is aware of Mr. Wilford's initial absence from the train and often questions the morality of Melanie's decisions. In addition to keeping the train running, Javi uses computer algorithms and hacked satellites to predict the environment surrounding and ahead of Snowpiercer. During the coup by First Class, Javi sides with the Folgers at first but eventually betrays them and rescues Melanie. Following the aborted coup and the successful Tailie revolution, Javi, Bennett, and Melanie try but fail to prevent Mr. Wilford from taking over Snowpiercer. After Wilford joins his Big Alice with Snowpiercer and takes control of both trains, Melanie departs on a mission to collect atmospheric data and model the re-warming of Earth. During a struggle to stop the trains in time to pick up Melanie as she returns, Javi is mauled by Wilford's dog, Jupiter, and Wilford can prevent the trains from stopping. Javi survives but is left traumatized and under Wilford's control. Throughout the third season, Javi battles post-traumatic stress disorder with the help of Sykes. He later joins Layton in going to New Eden where he serves as a member of the town council and he appears to be developing a budding romantic relationship with Sykes.
Mike O'Malley as Sam Roche: Snowpiercer's lead Brakeman who is a former police officer and Wilford's security officer before the Freeze. As the train's chief law enforcement officer, he tried to maintain the train's order, but ended up respecting and allying with Layton, and ultimately decided to support Layton's revolution. He and his wife, Anne, and one of his three children survived the Freeze. In season 2, he works with Layton and his former subordinate Bess Till to maintain order in the wake of Wilford's return. He is one of Layton's leadership council alongside Ruth Wardell, Miss Audrey, and Bennett Knox following Melanie's departure from the train. Near the end of season 2, he is placed in the Drawers on Big Alice along with his family. After Layton's return in season 3, he is rescued by Till and Dr. Pelton, but they discover that Anne had died at some point during their time in the Drawers. Roche subsequently struggles to deal with his grief and anger over Anne's death. Roche and his daughter choose to join the people going to New Eden at the end of the season. At New Eden, Roche serves as the head of security and a member of the town council before mysteriously vanishing in a blizzard. Roche later reappears, revealing that he was captured by Milius' forces, but he managed to escape in the chaos started by Layton and Wilford.
Annalise Basso as Lilah Jr. "LJ" Folger-Osweiller (season 1–3): The isolated teenage daughter of Lilah and Robert, she lives uptrain with her parents in First Class, surrounded by luxury. Appearing to be quiet and diligent, she is later revealed as a murderous psychopath and sadist, and the mastermind behind the serial murders on Snowpiercer committed by her bodyguard and boyfriend Erik Sotto. She is exposed by Layton, arrested, and found guilty in a tribunal, but her acquittal on Melanie's order makes her one of the most hated First Class train passengers. Following the death of both of her parents in the revolution, the orphaned and defenseless LJ is evicted from First Class by Pike and is now surviving in Third Class with Oz, both of them hated by the train's passengers. LJ reluctantly takes on a janitorial job and bonds with Alex Cavill. She eventually starts dating Oz. In season 3, she and Oz run the Night Car together and continue their relationship. In "The Last to Go", the two get married. In "The Original Sinners", after Oz leaves her for New Eden, LJ accidentally chokes to death on her father's glass eye.
Jaylin Fletcher as Miles (season 1; guest seasons 2–3; recurring season 4): A Tailie child, whose parents and sister were left behind to die in the Freeze when their refugee group invaded the train, leaving him to be raised primarily by Andre and Josie – the rest of the Tail refer to him as "Miles and Miles" (implying no one is aware of his surname). Whip-smart and talented, his brilliant intelligence wins him a coveted apprenticeship that allows him to move up the train to Second Class. He is later fast-track appointed an Engine Apprentice by Melanie. He plays a pivotal role in the revolution, revealing the truth about Melanie's deception to LJ Folger and working with Bennett, Melanie, and Layton to kill the Folgers and their army by detaching seven cars from the train. In season 2, Miles is revealed to be working on engineering life systems and dreams of being one of the first people to recolonize the planet. In season 3, he visits Josie, is revealed to have continued his apprenticeship, and suggests the solution for settling the dispute between Layton and Pike. When the trains separate, Miles remains with Snowpiercer to continue training as an Engineer with Melanie and Bennett, resuming working in the Engine with them in the absence of Javi. In season 4, Miles serves as the sole Engineer on Big Alice following the escape from the Silo and settles in New Eden with his adoptive parents. Gabriel Jacob-Cross portrays young Miles during flashbacks in season 1 while Ezra Benedict Agbonkhese portrays him in season 4.
Steven Ogg as Pike (seasons 2–3; recurring season 1): A Tailie, who was a career convict, serving time in Cook County Jail for armed robbery at the time of the Freeze, but escaped. He is respected as a hardened and battle-scarred leader in the Tail, and a warrior of his people, though he often acts as a renegade, out for whatever he can get and has shifting loyalties. In season 2, he starts an illicit trade between the two trains which Layton agrees to allow under his supervision. After assassinating Terence on Layton's orders, Pike suffers something of a breakdown out of guilt. In season 3, he assists Ruth Wardell in running the resistance on Snowpiercer in Layton's absence. After attempting to assassinate Layton, Pike is stabbed to death by him in a fight to the death in "Born to Bleed". He subsequently reappears as a priest in Layton's coma dream in "Ouroboros".
Rowan Blanchard as Alexandra "Alex" Cavill (seasons 2–4; guest season 1): Melanie's daughter and Big Alice's Engineer, who was believed to have died in the Freeze when her mother was forced to leave her behind when Snowpiercer left Chicago seven years ago. In the season 1 finale, Alex returns aboard Big Alice. She spent seven years on Big Alice as Wilford's dedicated protégée. Wilford had also manipulated her into hating Melanie, her mother, by convincing her that Melanie "stole" the train and left her behind. Upon their reunification, Alex and her mother slowly began to rekindle their relationship. Alex also starts becoming friendly with LJ Folger despite their differing loyalties. Alex eventually abandons Wilford and joins Layton's pirate train. After reuniting with her mother in season 3, she decides to join Layton in going to New Eden rather than remaining on Snowpiercer with Melanie. In season 4, it is revealed that Dr. Nima Rousseau, the man who ended the world, is her biological father, having served as a sperm donor for Melanie. Grace Sunar portrays young Alexandra Cavill during flashbacks in seasons 1–3.
Sean Bean as Joseph Wilford (seasons 2–4; uncredited season 1): An eccentric billionaire and genius inventor, Mr. Wilford is the powerful creator of Snowpiercer, worshipped as a Messiah-like figure who used his company, Wilford Industries, to refit his luxury liner train, "Wilford's Dreamliner" and its global railway into an ark, Snowpiercer, to save a small population from the Freeze (though in truth Melanie performed the engineering work to do so, and he claimed the credit). Initially believed to be the mysterious and reclusive Head Engineer of the train represented solely by Melanie, it is later revealed that he was not actually aboard Snowpiercer, with Melanie only maintaining the illusion that he was. After the secret was revealed, wealthy passengers from First Class attempted a coup, and the Tail and Third Class began a revolution. In the season 1 finale, after the revolution, Wilford returns in control of a secondary supply train, Big Alice, and docks with Snowpiercer. Throughout Season 2, Wilford's actions result in him gaining support from Snowpiercer's passengers, eventually retaking control after fixing the train. In the end, he remains in control of Big Alice and the rest of Snowpiercer after Layton creates a Pirate Train to rescue Melanie. In Season 3, Layton regains control over Snowpiercer and Big Alice, confining Wilford to Big Alice's Brig. Following an attack, Wilford is transferred to Snowpiercer's Library. During Layton and Melanie's Civil War, Wilford escapes, gathers his remaining supporters, and attempts to regain control. However, Layton and Melanie join forces to stop Wilford, exiling him from Snowpiercer and sending him off on a track scaler. A year later, Wilford is revealed to have been rescued by the International Peacekeeping Forces and working in their bunker. In "A Moth to a Flame", Wilford is cornered by Layton and, with no way out, he commits suicide by smoking a poison blunt.
Chelsea Harris as Sykes (seasons 3–4; recurring season 2): Mr. Wilford's head of security aboard Big Alice, Sykes keeps her secrets and is loyal to Wilford. At the end of the second season, Sykes is captured by Bennett Knox and remains a prisoner onboard Layton's pirate train. In season 3, she has joined the crew, stating that she is doing her part as a prisoner of war. However, her loyalty to Wilford appears to be wavering and she is left to roam free by Layton after he retakes Snowpiercer. She later bonds with Javi over their mutual trauma from dealing with Wilford, revealing that her scars also come from an attack by Wilford's dog Jupiter. When the trains separate, Sykes joins Layton in going to New Eden. In season 4, she is running food production in New Eden and appears to have a budding romantic relationship with Javi.
Archie Panjabi as Asha (season 3): A survivor discovered by Layton in a nuclear power plant in North Korea. In "Born to Bleed", Asha tells Pike that she's Indian and has lived in England. In "Setting Itself Right", she sacrifices herself to save Snowpiercer from a cloud of toxic volcanic gas.
Clark Gregg as Admiral Milius (season 4): The leader of the International Peacekeeping Forces who pose a threat to Snowpiercer and New Eden. In "A Moth to a Flame", he is overpowered by Wilford and left to freeze to death on a frozen level, Wilford having received Headwood's cold treatment.
Michael Aronov as Dr. Nima Rousseau (season 4): A brilliant chemist and former colleague of Melanie Cavill's from MIT who leads an international research team dedicated to ending the Freeze. In "A Moth to a Flame", Nima is revealed to be the man who created CW-7 and caused the Freeze in the first place. In "Dominant Traits," he is revealed to be Alex's biological father, having served as a sperm donor for Melanie. In the series finale, Nima dies after deliberately exposing himself to the Freeze that he caused upon finally recognizing and understanding the mistakes that he had made.