Devil in Disguise

The Devil is an evil, sneaky fellow who sometimes likes to go incognito among the living to manipulate people and stir up some sin, or maybe just tempt some mortals and secure a few more souls for himself. Alternatively, maybe he has simply sent an agent of some sort to do his dirty work for him. Either way, there's a demon hiding among the living and this time, he may well have a less obvious name than Louis Cypher.

Super-Trope to Louis Cypher, which is a trope where he leaves rather obvious hints as to his identity.

Anime and Manga

 * Xellos from Slayers. Among his Exclusively Evil kind he's the most powerful one not counting godlike Dark Lords, but prefers to use manipulation rather than violence. Two most powerful Dark Lords died from consequences of one of them employing Xellos against the other—the second strongest managed to injure him in one-on-one battle, but he still got away. He's also honorable in his own way and can be a good company when he wants. In his world few humans have thorough knowledge of demonology, so he walks among them under his own name and title.
 * Not to mention that his self-proclaimed title is "Mysterious Priest". If people assume "Priest" always means "user of White Magic", it's their fault.
 * from Baccano! is the demon who gave the alchemists immortality taken human form.

Comic Books

 * In the Superman comics in the early 90s, it was revealed that publisher Colin Thornton, who had hired Clark Kent away from The Daily Planet to serve as editor for Newstime, was a mortal disguise used by the demon Lord Satanus.
 * Welcome to Tranquility has Funky Freddy Beasel, a seemingly ageless radio DJ who turns out to be a satanic demon known as The Host.

Film

 * This trope was invoked in The Last Temptation of Christ, during Jesus's final temptation while on the cross. A young girl introduces herself as an angel, says that Jesus has done enough, and offers him an escape from the cross. This boy accompanies as a friend him through a happy life with a wife and children. But this friend is actually Lucifer, and these years after escaping the cross were All Just a Dream to tempt Jesus into abandoning his role as Sacrificial Lion. As soon as Jesus rebukes this temptation, the illusion ceases and he instantly returns dying on the cross, accepting it as his destiny.
 * John Milton in The Devil's Advocate.

Literature

 * In Susan Cooper's The Dark Is Rising, turns out to be The White Rider, one of the most powerful Lords of the Dark (the other being The Black Rider.)
 * Crowley from Good Omens. His immediate superior is Hastur, who works for Beelzebub, who, in turn, reports to Satan himself.
 * The wizard in The Wizard's Dilemma (Diane Duane's Young Wizards series) is "overshadowed" by the Lone Power (meaning, possessed by the Devil, basically).
 * Played in a wonderful subversion in Heinlein's Job: A Comedy of Justice when the protagonist finds out that the only friend he and his wife made through their ordeal was actually the devil himself. Not only had he shown him genuine mercy and friendship, he had turned down the bet with God, thinking it sick and cruel.
 * In the backstory of The Lord of the Rings, Sauron presented himself to the Elves of Eregion as 'Annatar', the Lord of Gifts, and with their help made the Rings.

Live-Action TV

 * In one episode of Quantum Leap, Sam discovers that the "Al" who has been advising him is a fake, and is actually the Devil. Or Was It a Dream?
 * Ultraman Nexus: Dark Zagi disguises himself as one of the human characters until his body is fully restored in the last episode.
 * In The Twilight Zone the Devil and his underlings usually appear in the guise of ordinary people. In "The Howling Man" he appears to be some poor guy who's been imprisoned by an apparently crazy monk, but when someone takes pity and releases him his horns and tail reappear.
 * In Netflix's Warrior Nun, says he is "An angel, yes.  I will bring the light and I will cast out the dark".  He turns out to be the Big Bad. While we don't yet know if he's Satan himself, that "bring the light" sure seems suggestive, as Lucifer means "light-bringer".

Oral Tradition, Folklore, Myths and Legends

 * In an old fairy tale from Piemonte (North-West of Italy) there's a tale similar to the one of Blue Beard, where the Big Bad is the devil disguised as a human who takes human girls to his castle (which hides the door to Hell in a secret room) to work for him. Furthermore his nose is made of silver.

Radio

 * Satan from Old Harry's Game disguises himself whenever he visits Earth (except when he's going to scare somebody).

Theatre
"Joe: Who are you? Applegate: I am quite a famous character, Mr. Boyd. I have historical significance, too. In fact, I'm responsible for most of the history you can name. (Later, a passing stranger asks him, "Are you anybody?" He replies, "Not a soul.")"
 * in Pippin.
 * Applegate from Damn Yankees. Referencing, of course, that particular apple, the first temptation of man; Lola is one of his newer ones.


 * Basil Bob in The Tempest Chronicles

Video Games

 * In Hellgate:London, the story's narrator and hero's sidekick Murmur turns out to be . To be fair, the name's a lot more subtle that Lou Cypher, and isn't likely to mean anything to players without a background in demonology mythology/Mega Ten fan.
 * In Phantasy Star 4, you are accompanied for the span of one dungeon by an adventurer named Seth. While his name doesn't really hint at what he really is, using the Talk command from the menu gives subtle hints, and his abilities (most, if not all of which were used in previous fights by ) are almost a dead giveaway if you are paying attention. "Seth" is also the name of the ancient Egyptian god of darkness, as well as the father of all mankind in Judaic and Christian mythology.
 * In Whacked, the host and producer of the titular Blood Sport game show is Satan. He doesn't look out of place with all of the other toon characters until he reveals himself when his show gets canceled. He invented the game show to attract malevolent souls to claim.
 * In The World Ends With You, The Hidden Secret Reports reveal that . Although technically, the TWEWY cast is not alive at all.

Web Comics

 * In Scary Go Round, it is revealed that  is actually the devil rather than the figure that the comic had presented as such.

Web Original
"Mr. Anon: Please allow me to introduce myself. I'm a Man of Wealth and Taste."
 * Mr. Anon from Open Blue is a case where the name isn't quite obvious. Indeed, it doesn't even seem to mean anything, and isn't even shorthand for Anonymous. The clue for Genre Savvy RPers, however, are his catchphrases, tendency to ask people to guess his name, and favorite activity of making deals with people.


 * from Tales of MU.

Western Animation
"Mrs. Bouvier: I swear, Monty, you're the Devil himself. Mr. Burns: WHA?!! WHO TOLD Y... oh, er, heh heh..."
 * In The Simpsons episode "Lady Bouvier's Lover", Marge's mother goes out dancing with Mr. Burns:

"Devil Flanders: Hey, Bart. Bart (nonchalant): Hey."
 * Not to mention the Halloween episode where it's revealed the Devil is Ned Flanders. "Always who you least suspect", indeed.