Arthas Menethil was noble, honorable, and righteous knight of the Silver Hand, and prince of Lordaeron, but after taking up the cursed runeblade Frostmourne, he became a malevolent, cold and ruthless death knight, led the Scourge in destroying Lordaeron, and merged with the Lich King.
Thunderclap was initially nice and friendly to Arlo after first meeting, but quickly reveals his real carnivorous nature after devouring the critter.
Obadiah Stane Initially was at first nice and friendly to Tony Stark, but later reveals his true nature where he actually wanted to permanently remain in control over Stark Industries.
Harvey Dent was loyal and honest ally and friend of Batman, but unfortunate accident, make that become a criminal as Two-Face.
Charles Muntz was nice to Carl and Russell, but then reveals his true nature with the intent to capture Kevin.
Sloan and Bree Blackburn were initially nice and friendly to Eliza Thornberry, posing themselves as zoologists, but later reveal themselves to actually be ruthless poachers.
Commander Lyle Tiberius Rourke was initially nice and friendly to Milo Thatch for most time, but later reveals his true nature with intent to get the Heart of Atlantis.
Eddy's Brother at first seemed to be loved brother to Eddy, but quickly reveals his true sadistic nature by mercilessly abusing his brother.
Prince Hans for most time was nice and charming to Anna, but later reveals that he never loved her, he was just manipulating her to take the throne for himself. He then locks her in the room to freeze.
Stromboli was at first nice to Pinocchio, but quickly reveals his true nature by locking him in cage.
Dawn Bellwether starts off being compassionate and understanding to Judy Hopps, but in the final climax, she reveals herself as the mastermind behind the savage attacks.
Miss Mirimoto started off appearing as a compassionate and understanding teacher to the Mystery, Inc. Gang, but revealed her evil plot to seize power over Tokyo and resurrect the Black Samurai.
Lotso initially acted like what his name implies towards Woody and the gang, but later reveals his true twistedly evil nature.
Henry J. Waternoose III initially was friendly to Sulley most of the time, but later reveals himself to be Randall's chief boss to Sulley and Mike after being convinced by Randall to assist him and learning about Boo as he saw her as a threat to his business.
Prof. Robert Callaghan was initially very friendly towards Hiro Hamada and his friends. However, after a lab accident ends with his daughter Abigail disappearing that Hiro and his friends near the climax of the film discovered, Callaghan, sets the building on fire - presumably killing Tadashi - and escapes with Hiro's Microbots. Later, Callaghan becomes known as the supervillain Yokai, seeking revenge on Alistair Krei, whom he blames for the accident.
Andrea Beaumount first seen as an innocent, sad woman visiting Gotham City after ten years and wants to start her relationship with Bruce Wayne all over, but is later revealed as the murderously vindictive Phantasm, seeking vengeance on the mob gang who threatened and murdered her father.
Alejandro initially acted to be charming and nice man, but reveals his true nature after eliminating Bridgette.
Scarlett was initially a nice and wise person, but reveals her true nature as the genius of evil after taking control of the control panel.
Ernesto de la Cruz was initially nice and friendly to Miguel, complimenting him and sharing with him passion and love for music, but later he reveals his true nature to Miguel after admitting his murder of Hector and stealing his fame.
Ben Ravencroft first appeared to be a charming author who wanted Mystery, Inc.'s help to find a diary belonging to his ancestor Sarah Ravencroft to she was a good woman who practice Wiccan magic not a wicked witch, but was revealed he was really using them to find the diary which reveals to be his evil ancestor's cursed spellbook, so he can become powerful and start ruling the world.
General Lunaris greets Della Duck and treats her like a friend, then he shot himself witha laser gun and turned evil and tells the Moonlanders that they are mighty and plot to invade Earth.
Cassandra for a long time was loyal, protective and supportive Rapunzel's bodyguard, handmaiden and friend, but she reveals her true, treacherous and malevolent nature when she take a moonstone away from Rapunzel and fulfilling her "destiny".
Jack Rabbit was best top agent of T.U.F.F. and good partner to Kitty Katswell, but reveal his greedy nature by draining Kitty's knowledge of T.U.F.F.'s secrets from her brain and selling them to Verminious Snaptrap in exchange for money.
Patty Pupe seem to be nice and friendly puppeteer and idol to Lucretia, but she is actually mean girl to everyone even for her own puppets.
Cozy Glow at first was friendly, shy, kind filly, who become helpful and wise student of School of Friendship, but later reveals her true malevolent and power-hungry nature wanting become powerful pony in Equestria.
Tangier at first was nice and friendly to Russell Ferguson, but reveals his true nasty nature by sabotating another photo session by putting hot sauce to Russell's food.
Creek at first was nicest, most friendly and positive Troll in all of Troll Village giving other Trolls advice, but then reveals his revealing treacherous nature betraying his own race and become Chef's assistant as long as he survives.
King Candy started off appearing noble and welcoming, but then revealed himself as the ruthless tyrant and psychopath he truly was and reveals that he is actually Turbo in disguise.
Tord at first was friendly towards Edd, Matt and Tom, but later revealed his true colours and decided to cause terrorism by revealing his robot.
Anakin Skywalker at first he was a Jedi Knight fighting on the Light side of the force and trained by Obi-Wan Kenobi, but by Emperor Paplatine joined to Dark Side of the Force and he become the nestest Dark Lord of Sith known as Darth Vader.
Fiona Fox was originally friendly towards Sonic and his friends, but later, she reveals her true nature by joining with Scourge the Hedgehog and starts to betray the Freedom Fighters, which included poor Tails.
Evelyn Deavor started out as a friendly, loyal, and kind-hearted lady who is eager to help others. She is also laid-back at times. Unfortunately, she later reveals her true nature when she unmasks herself as the real Screenslaver. During the confrontation, she revealed that she never supported her brother's mission in bringing supers back into public and plans on keeping them illegal forever.
Sentinel Prime was the original leader of the Autobots before Optimus until the war for Cybertron left him no choice but to form a partnership with Megatron. However, it becomes clear that he blackmailed Megatron so he could take over the leadership of the Decepticons for himself.
Jack Torrance was initially a good father and loving husband, but after descending into madness in the Overlook Hotel, he becomes a psychotic killer.
Dr. Sebastian Caine started out as a normal scientist, but after lying to the U.S. government and testing the invisibility serum on himself, he becomes a murderer.
Miranda Frost was initially an ally to James Bond after being sent by M, but later reveals that she was the one who betrayed James Bond and that she works for Gustav Graves.
In the beginning, M was first seen as a head of security who sponsored the League of Extraordinary Gentlemen who recruited various figures like Alan Quartermaine to stop the infamous Fantom from starting a world war, but was later revealed that he was the Fantom and was tricking the League so he can get the essence of each member for his plan to rule the world. And he was also later revealed as Professor James Moriarty.
Dorian Gray was first seen as a charming immortal who later joined the League of Extraordinary Gentlemen, but later revealed that he was a spy for M, sent to steal the essence of each member
Palpatine was introduced friendly, but during the climax of Revenge of the Sith, he is revealed to be Darth Sidious.
Warp Darkmatter was introduced as Buzz's partner, but after revealing that he is a double agent and the traitor of Star Command, he is one of Zurg's minions.
Simone Lenoir and Lena Dupree were first seen as innocent hostesses of a bayou island asking Mystery Inc. help solve a mystery there with Jacques as their ferry man, but were later revealed as monstrous werecats ought seek the life force of Mystery, Inc. so they can be immortal.
Monika was introduced as friendly and polite to protagonist. However, she reveals her true nature to protagonist and her real plans to eliminate Natsuki, Yuri and Sayori to be with the protagonist forever.
Makoto was introduced to be polite, cheerful and friendly towards others, including the Minato brothers and their sister, Asahi, but it soon revealed that he is an obsessive, egotistical and power hungry hero-wannabe who wanted to abuse the power of Ultraman Orb Dark to gain his fame as the "real" Ultraman.
Mizuki used to be friendly towards Naruto and gives him an "advice" to pass the exam, but it was revealed that he only used Naruto to manipulated him to steal the sealed scroll and give it to him just for being loyal to Orochimaru.
The A-Squad Rangers were introduced as reinforcements of S.P.D., but later betrayed Commander Kruger as they reveal themselves to be traitors of S.P.D..
Carrie White started out as a loner and an outcast student who is harshly treated by everyone at school and even her own mother Margaret White. Later, she becomes a psychotic murderer after the infamous blood prank during the school prom.
Andrew Detmer starts out when he is friendly towards his cousin Matt and his friend Steve. But later, he reveals his malicious nature to Matt during the final climax and calling himself an "Apex Predator".
Buddy Pine used to be an enthusiastic fanboy of Mr. Incredible who wants to become his sidekick named IncrediBoy. Years later, he become a supervillain named Syndrome who wants to get revenge on Mr. Incredible for rejecting him for all the years.
Bago Go started out as a friendly and cheerful alien who likes to sell illegal weapons to his customers, like his favorite one Adu Du. But later, he reveals his true egotistical and greedy nature when he and his brothers arrived on Earth.
Norman Osborn is introduced pleasant and kind, but after testing the serum on himself as well as driving to insanity, he becomes a supervillain known as the Green Goblin.
Eddie Brock is introduced as a photographer for the Daily Bugle, but after getting fired for forging a photo of Spider-Man robbing the bank as well as losing his girlfriend Gwen Stacy to Peter Parker and the symbiote lands on him, he becomes a supervillain known as Venom as he gets revenge on Peter for making him lose his job and his girlfriend Mary Jane.
Count Dooku starts out as a Jedi apprentice to Yoda as well as a Jedi Master to Qui-Gon Jinn, but later turned to the Dark Side and joined the Sith.
Henry Evans started out as a friendly and calm towards his cousin, Mark. Later, he reveals his cruel and sociopathic nature that he is indeed a murderer who wants kill his family no matter what it costs.
Clayton initially posed himself to be a kind (if somewhat impatient) guide to Tarzan, Jane, and Professor Porter, but later reveals to Tarzan while locking him up that he planned to cage all the apes from the very beginning and sadistically praises Tarzan that he owes it all to him.
Will Teasle first appearing as a well respected and kind sheriff, Teasle eventually shows his true nature when he arrests John Rambo for lack of both vagrancy and loitering (even though he was innocent) and lets his deputies abuse him to his breaking point which nearly led to the destruction of the Washington town of Hope.
While this TV pet show host may look like Happy Chapman lives up to his name, in reality, he is an arrogant, selfish, and greedy jerk who abuses his assistant Wendell, and is not satisfied with his job as a TV pet show host. His original star pupil was a famous cat named Persnikkity, whom he grew tired of due to his allergies and then puts Persnikkity in the city pound. Chapman wants a dog to star in his show so he can overshadow his more successful news anchor brother Walter in fame. So he steals Odie for his own gain and then uses a dreaded shock collar on him, and even laughs at the dog's pain from powerful jolts.
Darla Dimple initially appears as a sweet and darling little girl who loves animals, but reveals herself to be a complete egomaniacal psychopath and rotten to the core spoiled brat who wants the spotlight to herself and is afraid Danny and his animal gang will ruin it and is determined to stop them at all costs.
Gage Creed used to be a sweet and innocent child until his death. Later, he becomes a ruthless killer who was possessed by by the necromatic force Wendigo infested at Pet Sematary and was full of spite and cruelty.
Jenny first appeared as a friendly and shy woman who works as M.A.T.A.'s TEKNO member. Later, she was revealed to be Numeros' spy who worked with Uno.
Gwen Grayson first appear as the beautiful and friendly class president of Sky High who Will Stronghold has a crush on. But later revealed to be the villainous arch-nemesis of the Commander, Royal Pain, seeking revenge.
Baylor Hotner first appeared as an actor from Hollywood but rampaged the city as Crybaby Clown to perfect his role in his new movie.
Ted Dymer started as a friendly toy store owner, but Batman and Simon Trent learn that Dymer is the Mad Bomber from an episode of the Gray Ghost.
Rex Dangervest was friendly, helpful and nice to Emmet, who want help him to stop Armamageddon but he reveals his identity to Emmet and he planned cause the Armamageddon to get revenge on his old friends for abandoning and forgetting him.
Baldi initially appeared to be nice teacher towards the player and helps the player solve the math problem for the player. However, after the player get an answer wrong, Baldi revealed his true nature when he was about to "punish" him for getting an answer wrong.
Hal Stewart started out as a cameraman who works with Roxanne Ritchi, whom he falls in love with. Later, he turned into a ruthless supervillain known as Tighten to get revenge on Megamind for tricking him into believing he was destined to becoming a hero, as well as stealing Roxanne Ritchi from him.
Malty S. Melromarc initially appeared to be sweet and cheerful young woman who wanted to join Naofumi's party. Later, she stole all of Naofumi's equipments and framed Naofumi for attempting to rape her, turning him into an unwanted criminal.
Jeff used to be an innocent boy who always hanging out with his brother Liu. On their way to school, they are bullied by Randy, Troy, and Keith until Jeff accidentally injured Keith and Troy. Later, he was burned on the face by Keith by light a fire on a bleach. Because of this, he becomes a psychotic serial killer known as Jeff the Killer.
Eren first appeared to be a heroic and vengeful due to holds a personal vendetta against all Titans and his goal is to wipe them all off the face of the Earth. Later, he becomes more of an antagonistic anti-hero to a dangerous villain as he threatening to undermine the national security of Eldia in the face of an impending war against the world due to his assertive and arbitrary actions, as he commits numerous villainous acts.
Jim Starling was proud and well meaning actor playing as Darkwing Duck, but by his morbid jealousy, that his role was taken away by Drake Mallard in result he lost his mind and become a monstrous villain named Negaduck.
Esther initially appeared as a kind girl which leads the Colemans to adopt her to their family. However, she is revealed to be a serial killer who wants to murder families for her own satisfy.
Light used to be a boring intelligent high school student with nothing to do with his life. He later received Death Note and began to used it to become a god of a new utopian world.
Doofus Drake initially appeared to be friendly towards Louie, but later reveals that he is actually insane spoiled brat.
Scrooge McDuck's board of directors were first seen as stiff board members more cheaper than Scrooge hired by Scrooge to help keep McDuck Enterprises up and running, but were later revealed to be the High Command, the head of the evil organization known as F.O.W.L.