Atlas/Frank Fontane - Bioshock
In Bioshock, Atlas, later revealed to be Frank Fontaine, betrays the player by manipulating them throughout the game. Posing as a friend and ally, Fontaine uses the protagonist to further his own goals and seize control of the underwater city of Rapture.
Kendra - Dead Space
In Dead Space, Kendra Daniels, a seemingly helpful character, is revealed to be a traitor when she attempts to escape with the mysterious Marker. Her betrayal leads to her own demise, as she becomes a victim of the monstrous Hive Mind.
Albert Wesker - Resident Evil
Albert Wesker, a recurring antagonist in the Resident Evil series, betrays his teammates in the Special Tactics and Rescue Service (S.T.A.R.S.) unit. As a double agent for the Umbrella Corporation, Wesker manipulates events to further the company's sinister goals, making him a central villain throughout the series. During the game, we meet up with Bravo team leader Enrico, who reveals that there is a traitor among the group. However, before he can reveal the name, he is killed. It's not until near the end that we learn Wesker set the whole thing up, using his own S.T.A.R.S. in order to collect combat data. He is eventually killed by the Tyrant, but he manages to escape thanks to the injection of the special T-Virus.
The Gods - God of War
In God of War, the gods betray Kratos, the game's protagonist, by manipulating him into killing his own family. This act of deception sets off a chain of events that lead to Kratos' quest for vengeance against the gods, ultimately shaping the narrative of the series. Kratos formerly served the Gods until they tricked him into killing his own family. Blinded by rage, Kratos vows to take revenge against the Gods by killing them one by one.
Tanya - Mortal Kombat 4
Tanya, a character in Mortal Kombat 4, betrays her allies by siding with the villainous Quan Chi and Shinnok. Her deception results in the imprisonment of several heroes and the temporary fall of the Earthrealm, earning her a reputation as a cunning and untrustworthy adversary.
Pigma Dengar - Star Fox 64
In Star Fox 64, Pigma Dengar, once a member of the Star Fox team, betrays his comrades by joining forces with the evil Andross. His treachery leads to the death of Fox McCloud's father, James, and sets the stage for a series of intense confrontations throughout the game. Never trust a pig - something Fox's father learned the hard way. A former member of the original Star Fox team along with Peppy Hare and James McCloud, they were sent to investigate the planet Venom. Not long after they arrive, Pigma betrays the team, allowing Andross to capture them. Luckily, Peppy escapes, but James wasn't so lucky.
Yeah, Pigma was just awful. Poor James.
Thankfully, it was good shooting Pigma down for good.
"Your daddy screamed real good before he died!"
General Shepherd - Call of Duty: Modern Warfare 2
General Shepherd, a key character in Call of Duty: Modern Warfare 2, betrays his own team, Task Force 141, in a shocking twist. After using them to eliminate his enemies and consolidate power, Shepherd kills his own soldiers to cover his tracks, making him a primary antagonist of the game.
Sonny Forelli - Grand Theft Auto Vice City
Sonny Forelli, a character in Grand Theft Auto: Vice City, betrays the game's protagonist, Tommy Vercetti, by setting him up in a drug deal gone wrong. This act of treachery ignites a violent feud between the two, culminating in a final showdown in the game's climactic conclusion.
Ada Wong - Resident Evil 2
Ada Wong, a character in Resident Evil 2, betrays both the protagonist, Leon S. Kennedy, and the Umbrella Corporation. As a double agent, Ada manipulates Leon throughout the game, ultimately seeking to obtain a sample of the G-Virus for her own mysterious employers. Leon first meets Ada in Raccoon City. She claims that she is looking for her boyfriend. Leon, being the good guy that he is, believes her. He later meets up with Annette Birkin, who informs him that Ada is a spy. Leon doesn't believe her until Ada tells him her true purpose was to get a sample of the G-virus. However, she eventually dies, or so Leon thinks.
Big Smoke - Grand Theft Auto: San Andreas
Big Smoke, a character in Grand Theft Auto: San Andreas, betrays his childhood friend and protagonist, Carl "CJ" Johnson. Aligning himself with the game's antagonists, Big Smoke becomes a major enemy to CJ and the Grove Street Families, ultimately leading to his downfall. Smoke was always helping you at first, but later... CJ saw that he was helping CRASH and the Ballas gang.
The Newcomers
? Cortana - Halo
In the Halo series, Cortana, the AI companion of the protagonist Master Chief, betrays him in Halo 5: Guardians by revealing her plan to impose her own vision of order on the galaxy. Her actions force Master Chief and his allies to confront her and stop her from achieving her goal.
? Wheatley - Portal 2
Wheatley, a seemingly friendly AI in Portal 2, betrays the protagonist, Chell, after gaining control of the Aperture Science facility. Driven mad by power, Wheatley becomes a primary antagonist, forcing Chell to confront and defeat him in order to escape. Wheatley is the dumbest moron who ever lived, but you know what? I like him that way.
The Contenders
Daina Le Guin - Dead Space 2
In Dead Space 2, Daina Le Guin betrays the protagonist, Isaac Clarke, by manipulating him into activating a Marker, a powerful and dangerous alien artifact. Her betrayal is revealed when it becomes apparent that she intends to use the Marker for her own nefarious purposes. Daina is the woman who helps Isaac through the first third of the game. She tells Isaac she has a cure for his dementia, and he travels to her at the top of the church. She acts mysteriously, but Isaac doesn't believe he would be betrayed yet again. Well, he does. Daina is a Unitologist who wants to spread convergence everywhere, killing everyone, by making Isaac create more markers. There's no cure, and she basically laughs at him. Luckily, she is ripped apart by a gunship seconds later.
Magolor - Kirby's Return to Dreamland
In Kirby's Return to Dreamland, Magolor, initially portrayed as a friendly character seeking help, betrays Kirby and friends after they aid him in restoring his ship. Magolor's true intentions are revealed when he uses the ship to seize control of the universe, forcing Kirby and his friends to stop him. I would have never expected this much of a plot twist from a KIRBY GAME.
Ancano - The Elder Scrolls V: Skyrim
Ancano, a character in The Elder Scrolls V: Skyrim, betrays the College of Winterhold by attempting to harness the power of the Eye of Magnus for himself. His actions put the college and its inhabitants in danger, forcing the player to intervene and stop his power-hungry plot.
Micah Bell - Red Dead Redemption 2
In Red Dead Redemption 2, Micah Bell betrays the Van der Linde gang, his own comrades, by working as an informant for the Pinkertons. His treachery leads to the gang's downfall, setting the stage for a series of confrontations and the tragic conclusion of the game.
N - Pokemon Black and White
N, a character in Pokemon Black and White, betrays his own father, Ghetsis, and the Team Plasma organization. Initially believing in their cause, N has a change of heart after battling the player, ultimately deciding to stand against their plans for world domination.
Dimentio - Super Paper Mario
Dimentio, a character in Super Paper Mario, betrays his fellow villains by manipulating events throughout the game. His ultimate goal is to seize control of the Chaos Heart, a powerful artifact, to reshape the universe in his own twisted image, forcing the heroes to stop him. Super Paper Mario is known for being one of the darkest Mario games ever (which is good, by the way), and what was needed to make it perfect was a plot twist that NO ONE would have expected.
Sans - Undertale
Sans, a character in Undertale, betrays the player in the Genocide Route by becoming an unexpected final boss. As the player ruthlessly eliminates all inhabitants of the Underground, Sans steps in to protect the last remaining monsters, defying expectations and challenging the player's actions.
Pokey Minch - Earthbound/Mother 2
In Earthbound/Mother 2, Pokey Minch betrays his childhood friend, Ness, by siding with the game's primary antagonist, Giygas. Driven by greed and selfishness, Pokey's actions lead him down a dark path and ultimately result in a confrontation with Ness and his friends.
Dickson - Xenoblade Chronicles
In Xenoblade Chronicles, Dickson betrays the protagonist, Shulk, and his friends by revealing himself as an agent of the game's primary antagonist, Zanza. Driven by a lust for power, Dickson's betrayal forces Shulk and his allies to confront him in a series of climactic battles.
Jack Krauser - Resident Evil 4
In Resident Evil 4, Jack Krauser betrays his former ally, Leon S. Kennedy, by working as an operative for the game's primary antagonist, Osmund Saddler. Driven by a desire for power, Krauser's actions lead to a series of confrontations with Leon throughout the game.
Vergil - DMC: Devil May Cry
Vergil, a character in DMC: Devil May Cry, betrays his brother, Dante, and their alliance by revealing his true intentions of taking over the demonic world. His pursuit of power and control forces Dante to confront and battle him, culminating in a climactic showdown.
Private Goomp, Sergeant Guy and Corporal Paraplonk - Mario and Luigi: Bowser's Inside Story
In Mario and Luigi: Bowser's Inside Story, Private Goomp, Sergeant Guy, and Corporal Paraplonk betray Bowser by secretly working for the game's primary antagonist, Fawful. Their treachery is revealed when they help Fawful take control of Bowser's Castle, forcing Bowser and the Mario brothers to join forces and stop the villains.
Lance Vance - Grand Theft Auto: Vice City
Lance Vance, a character in Grand Theft Auto: Vice City, betrays the protagonist, Tommy Vercetti, by siding with the game's primary antagonist, Sonny Forelli. Driven by greed and a desire for power, Lance's actions lead to a climactic confrontation between him and Tommy.
Marx - Kirby Superstar
In Kirby Superstar, Marx betrays Kirby and the people of Popstar by manipulating Kirby into summoning the powerful Galactic Nova. Marx seizes control of the wish-granting Nova, seeking to conquer the universe, which forces Kirby to battle him and restore peace.
Gary - Bully
Gary, the primary antagonist in Bully, betrays his former friend, the game's protagonist Jimmy Hopkins, by spreading lies and turning the school's various cliques against him. This act of treachery sets the stage for Jimmy's efforts to regain control of the school and confront Gary.