Go-To Alias

Whether it's for serious reasons or a silly scenario, occasionally a character will have to pretend to be someone he isn't. As the story continues (probably as a treat for people who've been watching the show for some time), he'll use a name that has been mentioned in the past. This is the Go-to Alias: a default pretend name on which the character falls back when he's pretending to be someone else.

A variation on this would be the Go-to Identity, when this name carries a history or personality to which the character sticks whenever he uses it. In more dire circumstances, the character will start associating with that identity more than his own. If Played for Laughs, he will be Lost in Character.

If the character uses multiple aliases with a common thread, they're Themed Aliases.

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Comic Books

 * Batman: Alfred Pennyworth often uses his middle names (Thaddeus Crane) as an alias when he goes undercover.
 * Wolverine uses the identity of "Patch" (wearing an eyepatch), a mercenary, when he acts undercover in the Far East.
 * In Knights of the Dinner Table, 'Weird' Pete's go-to alias is John Mephisto; the name of an old HackNoia character of his.

Literature

 * In the James Bond novels by James Gardner, Bond often uses the alias of 'James Boldman'.
 * The Saint: Simon Templar liked to use the alias "Sebastian Toombs".
 * Lord Peter Wimsey generally uses his two middle names (Death Bredon) as a Nom De Guerre.
 * In the Company Z novels by J. T. Edson, Alvin Fog uses 'Rapido Clint' as his alias whenever he poses as a criminal.
 * Locke Lamora and Jean Tannen have had a lot of aliases in their time... but when pressed for an identity on short notice, they can always fall back on Tavrin Callas. (This is sort of a prank on their part; the first time Jean used that name, he was infiltrating the cult of the death goddess and faked his own suicide. They figure if anyone traces the name far enough back, the followers of the death goddess can declare it a miracle.)
 * In the Lucky Starr series, Lucky's go-to alias is William Williams.
 * Author Gordon Korman used "G. Gavin Gunhold" as a running alias across multiple books. Gavin takes on a life of his own more than usual in The Wizzle War (as the world's most perfect, absentee student) and A Semester In The Life Of A Garbage Bag (an obscure Canadian poet) in particular.
 * Donald E. Westlake's John Dortmunder character is a career criminal. He has occasionally used the alias "John Diddums" (he claims to anyone who asks that it's Welsh), a name he dislikes but uses involuntarily in circumstances that preclude using his real name.

Live Action TV

 * Friends:
 * Phoebe Buffay is practically guaranteed to use the name "Regina Phalange" whenever pretending to be someone else.
 * Joey Tribbiani apparently uses Ken Adams from time to time, but we only see it in action in one episode.
 * Parks and Recreation:
 * Andy Dwyer used Bert Macklin, FBI agent, in a few scenarios. However, he eventually dies and is replaced by his brother, Kip Hackman. Or so the President's enemies think.
 * In the same vein, April Ludgate likes being Janet Snakehole. Originally just the daughter of the owner of the Snakehole Lounge (so she could score free drinks), the next time we see her she's a fabulously wealthy widow with a Mysterious Past.
 * Chuck:
 * The eponymous Chuck Bartowski introduced the idea of Charles Carmichael (the successful version of himself) in the third episode, "Chuck vs. the Tango". It isn't used again until the start of Season Two, and over the course of the season it becomes so associated with him that it ends up as one of his operating aliases. In one episode people are congratulating him as both "Bartowski" and "Carmichael" as he walks through a crowd.
 * Further down the line, even Morgan gets into the act, establishing Michael Carmichael as his go-to alias.
 * On The Office, Michael Scott has Agent Michael Scarn. Originally his go-to improv character, eventually the main character of a script he writes and, over the course of several years, films.
 * On Doctor Who, the Doctor often goes by the generic "John Smith" when forced to give up a name or being "under cover".
 * On Will and Grace, Karen's alias was "Anastasia Beaverhausen".
 * Burn Notice's Sam Axe always, always uses the name "Chuck Finley" when he needs an alias for the Job of the Week.
 * After the season 4 in-series proclamation that "Chuck Finley is forever", there is official merchandise for Sam's go-to alias. Mr. Finley is on the way to becoming a Memetic Badass, a la Wang Fire of Avatar fame.
 * Seinfeld:
 * George Costanza had Art Vandelay, who was an architect. Usually he used this alias when he didn't need to.
 * Cosmo Kramer would often pretend to be H.E. Pennypacker or Dr. Van Nostrand.
 * Jerry himself occasionally went by Kel Varnsen.
 * In the Puerto Rican Day episode, all three appeared to bid on the same apartment.
 * In Plain Sight gives us Mary Sheppard and Marshall Miller.
 * On Modern Family, Phil uses Clive Bixby, designer of high-end electro-acoustic transducers, when roleplaying with Claire, who uses the name Juliana for the same.
 * Winston from New Girl uses "Theodore K. Mullins", Nick/Schmidt's lover on the down low.

Tabletop RPG

 * Classic Traveller, The Journal of the Traveller's Aid Society #16. Jon "Fast Johnny" McRae is a highly skilled and daring interstellar con artist. One of his favorite aliases is Commodore Laruskaa Korkoran of the Imperial Navy. He has gone to great effort to make it a solid identity that can't be easily exposed.

Video Games

 * The Suikoden series has the recurring alias of Scholtheim Reinbach III/IV. Made much funnier when one of the games actually features the real Scholtheim Reinback III.
 * In Planescape: Torment, The Nameless One has the option of using the moniker Adahn when talking to people.

Western Animation

 * On King of the Hill, Dale likes to go by Rusty Shackleford, generally whenever he's doing something sneaky, although he also refuses to sign his real name to any document, being a comical Right-Wing Militia Fanatic. At one point in the series, the actual Rusty Shackleford shows up--Dale knew him as a kid and had thought Rusty had died--it turns out that Rusty had just moved away and wasn't happy being connected with Dale's various acts of stupidity.

Comic Books

 * Batman has his Matches Malone identity that he uses to get information about the criminal underworld.
 * And Robin (Tim Drake) has Alvin Draper.

Literature

 * Miles Vorkosigan spends a lot of time under the alias Admiral Miles Naismith. Part of the series delves into him Becoming the Mask.
 * The Stainless Steel Rat, in his criminal days, had a constructed identity complete with false fingerprints, pads that changed the shape of his face, and so on, which he would adopt before beginning any con (and keep up underneath any other alias required for the con), so that the police would spend their time looking for that person instead of the real him.

Live Action TV

 * Leverage: Sophie Devereaux is the alias by which we know Gina Bellman's con artist.
 * Not that the rest of the gang doesn't have standard cover identities. John Rogers says it's to decrease the number of names they have to run by their legal team to keep them from slandering someone. A small selection:
 * Nate uses Jimmy Papadokalis every time he's playing a lawyer; he's used Tom Baker and Bob Gibson multiple times as well.
 * Eliot has used the name Wes Abernathy at least twice.
 * Parker has used Alice White at least twice.
 * On White Collar, Neal Caffrey uses Nick Halden as a regular alias when playing cons. Probably because he knows the FBI already knows about that identity; he's had others, but he'd prefer not to give up all his secrets.