Real Name as an Alias

A character needs an alias for some reason and technically doesn't lie about what their name is... it might be their real name if they're known by a nickname or vice versa, or their maiden name if they're a married woman, or their middle name being used as their first name, but the point is that the character can argue that it is their real name. Often used by disguised royalty with Overly Long Names, since they can just pick one of their many middle names to be called by.

Examples of Real Name as an Alias include:

Anime and Manga

In Monster, Nina Fortner uses her previous legal name, Anna Liebert, when giving a police testimony and obtaining marksmanship training.
Allelujah Haptism and Tieria Erde use their names as the start up code for their Gundams in Mobile Suit Gundam 00 while their fellow Meisters, Soran Ibrahim and Neil/Lyle Dylandy, use their codenames, Setsuna F. Seiei and Lockon Stratos.
FLCL's Haruhara Haruko- her name's revealed by Amarao to be Haruha Raharu.
During Ranma ½'s Martial Arts Rhythmic Gymnastics competition, female Ranma freely gives her full name, confident that nobody in attendance (which includes classmates from Furinkan High) will make the connection with her male side. She's right, and everyone blows it off as a bizarre coincidence. (Although Hiroshi and Daisuke would constantly pester Ranma about introducing them to "his sister" afterwards.)
Similarly, Kämpfer's Natsuru Senou enters the girls' half of his gender-segregated school under the name Natsuru Senou. Cue the characters getting confused between the male Natsuru or the female Natsuru, when they're actually the same person.
In Death Note, Light Yagami expends a lot of effort trying to learn the real name of "L", the Great Detective who opposes him. It's eventually revealed (although not in the manga or anime) that L is his real first name, which raises some Fridge Logic issues...
- Of course, Light never hears his last name, so he would have had to spend that effort anyway if he wanted to use the Death Note.

Comic Books

Batman's butler Alfred Pennyworth has sometimes used his middle names, Thaddeus Crane, as an alias when undertaking some activity he doesn't want linked to Bruce Wayne.
The Dark Phoenix Saga sees Jean Grey being seduced by a mutant named Jason Wyngarde, an aspiring member of the Hellfire Club. It turns out Wyngarde was one of their older and slightly more obscure enemies, the mutant villain Mastermind, and the X-Men just never found out his real name. Although he was also a Master of Illusion, and thus was able to disguise himself.
- Similarly, Mystique often posed as a secret service agent named Raven Darkholme in her early days. Turns out that's her real name. Amusingly, the Animated Adaptation of Days of Future Past (with some of Bishop's future thrown in) has a woman looking just like the Raven Darkholme identity, whom Mystique copied, tied up, but deliberately left alive to be a witness to "Gambit" assassinating Senator Kelly. Presumably, this came before the revelation that there was no "real" Raven Darkholme.

Fan Works

A number of fanfic writers have used the name of the actor who plays the character for an alias, crossing this trope with an Actor Allusion.
In Drunkard's Walk II, protagonist Douglas Quincy Sangnoir early on gives "Quincy Black" as his name to a 911 operator. ("Sangnoir" is French for "black blood".)

Film

When pretending to be a handmaiden in Star Wars, Queen Amidala used her lesser-known non-royal name, Padme Nabierre.
John McClane's wife at the beginning of Die Hard uses her maiden name, Gennaro. This provides a minor plot point when Hans Gruber later discovers John's real name, and doesn't realize the connection between John and Holly (for a while, anyway).
In 1937's Shall We Dance, the two leads, who are both famous under stage names, get married quietly in New Jersey under their real names.
The Disney Channel movie Motocrossed involved a girl posing as her brother to enter a motocross race, after he breaks his leg. The siblings have the same name (Andy, short for both Andrea and Andrew), allowing her a legal loophole: since she signed up as "Andy," instead of Andrew or Andrea, she could hold on to her win after she was outed.
Vicki Lester and Norman Maine in A Star Is Born get married under their real names by a town clerk who has no idea who they are.
James Bond does this quite often, though there are several occasions he uses proper aliases too. Somewhat justified in that officially he is a globetrotting employee for something called Universal Exports, which is actually a cover for MI6.
- In Tomorrow Never Dies he poses as a banker named James Bond and actually has a fake employment record, but is given away as a spy because it is too perfect to be true, and because he was asking too many questions, not because of his name.

Live-Action TV

Buffy the Vampire Slayer: When she runs away to LA, Buffy Summers calls herself "Anne," which is actually her middle name.
In an episode of Star Trek: Deep Space Nine, Garak tells Bashir a series of contradictory stories about a person named Elim, supposedly a high-ranking Cardassian official who either betrayed or was betrayed by Garak. Bashir later finds out that Elim is just Garak's first name, and when he confronts Garak and demands the truth, Garak says hat all the stories were true.
Christopher Perry Halliwell from Charmed called himself "Chris Perry" when he first introduced himself.
In The Cosby Show, lawyer Clair once introduced herself by her maiden name in order to speak on son Theo's behalf when he got ripped off by a T-shirt company.

Literature

In Katharine Kerr's Deverry novels, the character Nevyn (meaning "no one", and hence often taken to be a joke) uses his long-abandoned original name (Galrion) when he needs to be taken seriously by some nobles. Everyone who knows him assumes he just made it up.
In Robert Heinlein's Friday, the title character runs out of fake passports and falls back on the real one to pull a switcheroo with one of the fake IDs.
Lord Peter Death Bredon Wimsey uses the name Death Bredon in the Dorothy L. Sayers novel Murder Must Advertise. It's pronounced 'deeth'.
- By most people. Death Bredon just says it the way it looks.
In the novel The School Story, Zoe pretends to be a literary agent and uses her nickname "Zee Zee" together with a misspelling of her last name.
In Belgarath the Sorcerer, the eponymous character once used his old name Garath when working undercover. It lasted until he had to use magic to fend off an attack.
- As well, his daughter Polgara sometimes goes as the Dutchess of Erat, a title she technically owns although the duchy itself has been gone for thousands of years.
In The Truth-Teller's Tale, members of the royal family have secret names that no one outside the family knows. A prince in disguise uses his as an alias. The narrator is a Truth-Teller, who can detect when someone is lying, but since it's technically the prince's real name, she doesn't register that as a lie.
The L. M. Montgomery short story "The Pot and the Kettle" combines a double use of this with a Two-Person Love Triangle.
In The Ordinary Princess, Princess Amethyst works as a kitchen maid under her nickname, Amy. The king of the kingdom she's working in does the same thing--she knows the king's name is Algernon, but he uses one of his many middle names.
In How Not to Spend Your Senior Year, when Jo and her father fake their deaths and go into witness protection, she uses her middle name and her mother's maiden name to enroll at another school. When the federal marshals who put them in witness protection want her to use a completely fake name, Jo tells them about an incident when she and a friend switched names as a prank on a substitute teacher and couldn't pull it off because they weren't used to responding to someone else's name.
In The Goose Girl, the princess' full name is Anidori-Kiladra Talianna Isilee. When she goes into hiding, she calls herself Isi.
The prince in Sherwood Smith's A Posse of Princesses.
Miles Naismith Vorkosigan spent ten years running top secret operations under the cover identity of Admiral Miles Naismith. Once, on the point of being found out, he covered by pretending that "Admiral Naismith" was his clone, who used his Betan mother's maiden name because Betan law grants clones the legal status of family members.
In Spindle's End, when Katriona and Aunt take in the little princess, they take the last of her many names, Briar-Rose, and decide to call her Rosie.
In 1632, King Gustav II Adolf of Sweden decides to send cavalry to help out his time-displaced American allies, and despite the misgivings of his prime minister puts the 'notably headstrong and reckless' Captain Gars in command. It's only after Gars helps save the Americans from an attack that it's explained to them that his name is an acronym for Gustavus Adolphus Rex Sueciae - 'Gustav Adolf the King of the Swedes' in Latin.
In Monstrous Regiment, a regiment composed of women dressed up as men dress up as washerwomen to get themselves into a fort, and use their real names as their aliases.
In The Birthday Ball, Princess Patricia Priscilla dresses as a peasant and goes to the village school. When the schoolmaster asks for her name, she calls herself Pat. ("Quite a short name because I'm merely a humble peasant.")
In The Count of Monte Cristo, the aristocrat-born radical Nortier de Vilefort just goes by the name Nortier. In contrast, his conservative son calls himself de Vilefort. This is plot important, as the direct reason Dantes ended up wrongfully imprisoned, is because he unknowingly told de Vilefort about a letter that incriminated Nortier for treason.
In Andre Norton's The Prince Commands, Prince Michael Karl uses the "royalty with an overly long name" version while passing himself off as the American friend of the prince. His full name is "Michael Karl Johann Stefan Rene Erich Marie." So he calls himself "John Stephenson." It turns out he's not the only one using this, either.

Professional Wrestling

In WWE, Shawn Michaels' mid-2009 absence was explained by him taking the job of a ~~cook~~ chef at an office complex under the last name of Hickenbottom - that being Michaels' real-life last name.

Tabletop Games

Crossed with Significant Monogram in a classic Forgotten Realms character Lhaeo - Elminster's scribe (and butler, and apprentice) who was a great artist at building Jerkass Facade for Old Mage. "Coincidentally", this job made him good at digging through papers and turning people away politely, alert, ready for anything and acquainted either personally or from first-hand accounts with half or so of the most powerful people on the continent. There also was a missing prince of Tethyr, one Haedrak Errilam Alemander Olosar Lhorik... later also known as King Haedrak III. If his compatriots knew where he is before he was set to pull Rightful King Returns, he would receive about as much attention from assassins as Elminster gets from mad wizards. As it was, only one is known to get through (who was magically imprisoned, as seen in Forgotten Realms Comics).

Theatre

In Shakespeare's Henry V, King Henry goes undercover as a common soldier to learn the views and morale of his troops more closely. He gives his name to the soldier Pistol as "Harry le Roi" (Harry being a common nickname for Henry, and le Roi being French for "the King"). He says he's a Welshman - he was born in Wales.

Video Games

Frank, the first NPC you meet in zOMG!, is actually Labtech 123; no official statement has been made, but the general assumption is that "Frank" is in fact his real name.
- Pretty much confirmed by one of the H2k10 NPCs, who has mentioned him by name.
Played directly in Nine Hours Nine Persons Nine Doors: near the end of the game, it's mentioned that Clover's real name is... Clover.

Web Comics

Girl Genius had Gilgamesh Wulfenbach, aka "Gilgamesh Holzfäller". And this worked, even though, as this was aptly summarized -

Violetta: You mean to tell me that you knew this guy by name on Castle Wulfenbach, and it never occurred to you that this guy, with the same name, might be the mystery son the Baron had been hiding all these years?! IDIOT!

Western Animation

Foster's Home for Imaginary Friends: In the episode "Imposter's Home for Um... Make 'Em Up Pals" this happens with two characters: Goofball John McGee and John Larry McGee, Goofball's owner.
- Goofball is introduced by his first name. When Frankie overhears a group of kids calling him "John," she believes she has more proof he is not an imaginary friend. Goofball explains that it is his middle name and that he is named after his owner, Larry. He then tacks on another explanation that "Larry" is his owner's middle name.
In Teen Titans, Cyborg infiltrates the HIVE by donning a hologram to make him look human until he "powers up" into a rocklike form. He calls himself "Stone." Vic Stone is, of course, his real name.
- There's also Slade, whose real name is Slade Wilson. In the original comic his alias was Deathstroke the Terminator.
On Family Guy, Peter Griffin once tried to pull of a Line-of-Sight Name. The first three things he saw were a pea, a person crying one tear, and a griffin that flew in through the window.
In Young Justice, Artemis' real name is... Artemis Crock.

Real Life

British DJ Kenny Everett (real name Maurice Cole) once did a promo which ended with the words "...or my name isn't Maurice Cole!" The joke being that most of the listeners wouldn't have known his real name at the time.
Many famous women continue to use their maiden name after marrying, since they're already well-known and changing names might confuse fans. For example, J. K. Rowling continued to use her maiden name after becoming Jo Murray.
It's quite common to use one's first and middle name as a Stage Name. One prominent example would be Tim Allen (born Timothy Allen Dick). Another would be twins Nicholas Brendon Schultz and Kelly Donovan Schultz.