Obvious Judas

The work has a group get together for a common goal, usually to defeat a great evil, but there's just that one character who has a certain air about him. You just know that he'll eventually betray the group in some way or another, even though the writer has shown no evidence of this at all. The Trope Namer is obviously the Judas of the Christian bible who betrayed Jesus and the rest of his disciples.

Different from Obviously Evil in the sense that this character is one of the main protagonists to begin with, but just leaves the viewer/reader with the feeling that they'll be opposing the group at some point.

Compare Narrowed It Down to The Guy I Recognize, contrast The Mole.

Anime and Manga

 * Aizen from Bleach. Many a viewer said he acted "too nice" before he made his conversion into the main villain of the series. To the point where it was highly noticeable that it was all a facade.
 * Vegeta in the Majin Buu ark of Dragonball Z
 * Sasuke Uchiha from Naruto
 * Bizarrely, in the early arcs Sasuke (apart from, y'know, being a dick) had the highest proven teamwork-and-loyalty stats in the cell. He got them through the bell test by sharing his lunch, and he only didn't die for Naruto in Wave even as his Sharingan awoke because Haku was a nicer guy than he thought. Contact with Orochimaru and getting mind-raped by his  brother a second time seems to have set him off.
 * Defied: Hiei from Yu Yu Hakusho.
 * Yusuke's trust had a weird effect on Hiei. Later, in the Chapter Black arc everyone is startled when Hiei goes 'stop the tunnel to Demon World? Screw that, I want to go home,' and ditches the rest of the team. He comes back and saves Yusuke's life less than a week later, though. And then he and Hiei beat the snot out of each other for a while, and then they're friends again. Even after the team splits up.

Comics

 * Nack the Weasel from Sonic the Comic. So much so that Chaotix should have even seen it coming.

Fan Fiction

 * It has become incredibly common among writers of Harry Potter fan fiction to portray Peter Pettigrew (the Marauder who betrayed Harry's parents to Voldemort) in this fashion. It happens so often the phrase "JoePantoliano!Peter has come into use.

Film

 * Cypher from The Matrix. But then, he's being played by Joe Pantoliano.
 * Joe Pantoliano in just about everything, really.
 * Harry Osborne from the Spider-Man films. He was pretty much set up solely for the purpose of being Peter's Judas (starting with hitting on Mary Jane and starting to date her, the implication being that he wanted Peter to be jealous of him like he was jealous of his dad's affection for Peter).
 * Ephialtes from Three Hundred. Played for sympathy.
 * Magneto in X Men First Class. Conflicting views with Charles makes it obvious even to those who don't know the entire X-Men story that he will betray Xavier.

Literature

 * In The Last Hero the character of "Evil" Harry Dread proudly wears this as his hat, and even reminds the party that he is evil and therefore contractually obliged to betray them at some point. The heroes accept this as part of the Heroic Code and even congratulate him on a job well done when he does betray them.

Video Games

 * Kain from Final Fantasy IV. Mentioning killing Rydia due to an order from the king, clearly shows jealousy of Cecil at some points, and he's a Dragoon (one of the Darker and Edgier classes within Final Fantasy).
 * Cait Sith from Final Fantasy VII

Western Animation

 * Demona from Gargoyles
 * Eric Cartman from South Park. In fact, this trope is played with in many episodes.