Since two professors were announced for Pokémon Scarlet and Pokémon Violet in May, there's been some conjecture among fans that the professor in each game might have an antagonistic role in the opposite game.
The concept isn't a new one, as many characters can play a heroic role in one game and a villainous role in another. Often, the exact same game will even have different paths on which a given character's alignment differs. Regardless of what qualifies them as turncoats, these characters are all interesting to examine morality-wise.
Catherine
Catherine is one of two female protagonists of Catherine, and one of three main love interests in Catherine: Full Body. Her supernatural powers allow her to get away with a lot of questionable acts in the games.
Catherine is a game most commonly known for its ability to have both "bad endings" with Catherine and Katherine and good endings. The different routes make the game a very visual novel-inspired game. The girls can either go as far as killing Vincent or even marrying him, depending on the route. It all depends if the player plays their cards right.
Cole MacGrath
Cole MacGrath is one of the protagonists of the Infamous series. He gained the power of electrokenesis in an explosion and sets off on an adventure that tests his morality.
Throughout Infamous and Infamous 2, Cole experiences many opportunities to shape his morality. The decisions the player makes will shift Cole toward good or evil. Depending on Cole's morality shift, his lightning will also change color. A good Cole will have white/blue lightning, while an evil Cole will have red/black lightning.
Xander
Xander is one of the two main leaders of the families in Fire Emblem: Fates. The Nohrian prince is a powerful force to be reckoned with, but his morality changes greatly depending on the game.
In Fire Emblem: Conquest Xander is a loving older brother to his family that does his best to save people while appeasing his father's orders. However, in Birthright Xander is a monstrous and vengeful prince that follows his father's orders without question. Xander even kills his sister Elise in Birthright, something Conquest Xander would never think to do.
Maxie
Maxie is the leader of Team Magma, a group dedicated to the land. He functions as Pokémon Ruby's main antagonist and one of the villains in Pokémon Emerald.
Pokémon is an incredibly beloved series, and few games are as beloved as Ruby and Sapphire. Fans generally remember Maxie's role in Ruby but may not remember that he was also in Sapphire. However, in Sapphire, Maxie and Team Magma aren't villains. They're simply trying to stop Archie and Team Aqua, and will even assist the player.
Archie
Archie is the fearless leader of Team Aqua, a team devoted to the sea. However, the extremism (or lack thereof) of their devotion depends on the game the player has chosen.
Archie is a scary boss battle that can end challenge runs like Nuzlockes in a flash with his Sharpedo. However, the iconic villain is actually only a villain in Sapphire. The reverse of his counterpart Maxie, Archie is a supporting cast member in Ruby who does his best to help the player take Maxie and Team Magma down.
Byleth
Byleth is the protagonist of Fire Emblem: Three Houses and a minor antagonist in Fire Emblem Warriors: Three Hopes. In the latter game, they can also be a recruitable ally.
Byleth is interesting because their morality is changeable in both games, while they are the protagonist and antagonist in the other. This gives Byleth a unique position in fiction. They can either be a good protagonist, evil protagonist, evil antagonist, or good antagonist. They're one of the most customizable characters in any video game, from their looks to their morals.
Alternis Dim/Ringabel
Two of the main characters in Bravely Default are actually the same person from two different dimensions. One is the Dark Knight Alternis Dim, the other is the beloved hero of light Ringabel.
Throughout Bravely Default, more and more versions of Alternis are discovered. It turns out that the character's alignment can change drastically depending on the dimension, with some Alternsis's helping or hurting more than others. Only one thing remains consistent across all Ringbals and Alternis: their love for Edea.
Starkiller
Starkiller is The Force Unleashed's protagonist and Darth Vader's apprentice. Starkiller starts out as a sith no matter what but has options they can take towards the end of the game.
The Force Unleashed is one of the best Star Wars games out there, and Starkiller is a big part of the game's narrative success. At the end of the game, Starkiller is presented with two options. First, they can join the light side by helping Vader and defeating Emperor Palpatine. Or, they can choose the dark side by helping Palpatine kill Vader.
Dimitri
Dimitri is the leader of the Blue Lions and one of the best characters in the Fire Emblem series. He's best known for his intense rivalry with (and eventual hatred for) his relative Edelgard.
Dimitri is one of the best characters in both Fire Emblem Warriors: Three Hopes and his original game, Fire Emblem: Three Houses. The multi-route options of both games give Dimitri the chance to either align with the protagonist or go down a much darker road. Depending on the route, Dimitri can either be the player's most trusted ally or most dangerous opponent.
The Skyrim Player Character
The Skyrim protagonist is one of the most customizable characters in any game. Players can do pretty much anything they want with their character, including warping their morals and alignment.
While it won't be as obvious as in some other games, some questlines in Skyrim are certainly of dubious morality and can take the player character down a slippery slope. Games like Infamous may have big glaring screens that tell the player they're being moral or immoral, but Skyrim players will have to decide that for themselves based on their actions.