Only Known by Initials

Some people are Only Known By Their Nickname, others are on Last Name Basis, while some people are primarly known to the public by their initials or their initials plus their last name. The reasons may vary. Perhaps they have an Embarrassing First Name they'd rather not use, and maybe they have an Overly Long Name that needs to be abbreviated. Others may think it sounds classier, and some may prefer the fact that it is more gender neutral.

This was particularly common among Real Life English male writers of the early 20th century, especially academics like A.J. Ayer (philosopher) and A.J.P. Taylor (historian). You do still find some English writers who go by initials today, possibly because of the resulting intellectual associations - like A.N. Wilson or D.J. Taylor.

It's also sometimes found with Real Life female writers, often to hide (or at least make less obvious) the fact that they are female.

Anime and Manga

 * C.C. from Code Geass (its pronounced C-two)
 * The same goes for V.V.
 * TK, In the dub version of Digimon, (his full name is Takeru Takaishi, in the Japanese, at least).
 * Angel Beats has the American TK.
 * T.R. (Thomas Riley) Edwards from Robotech. Some sources erroneously assume that he has a hyphenated last name (it's true that Riley is usually a last name). However, if this was the case, he would be properly known as T. Riley-Edwards.

Comics

 * BA (Boris Alphonso) Felton from Knights of the Dinner Table
 * TC from Preacher

Film

 * DC from That Darn Cat is only once referred to by a censored version of his full name, not counting the title, and is never referred to by his real full name, on account of it being profane.
 * Tenacious D in The Pick of Destiny: JB (Jack Black) and KG (Kyle Gass). Pronounced Jables and Kage.
 * L. B. "Jeff" Jeffries from Rear Window.
 * H.W. in There Will Be Blood
 * Played for Laughs in Down With Love. When Barbara meets the high-ups of the company publishing her book for the first time, they introduce themselves as P.G., C.B., C.W., J.B., J.R. and R.J. When Barbara replies with "Okay!", P.G. says that O.K. couldn't make it, he and T.B. are having breakfast.
 * T.S. Quint from Mallrats

Literature

 * PK Pinkerton in The Western Mysteries, is only known by the initials PK or by the nickname Pinky, as part of the author's attempt to create Ambiguous Gender.
 * Subverted with Mr. A.E. Pessimal from Thud, as this is actually his full name.
 * Superman: Last Son of Krypton. One of Lex Luthor's henchwomen is named Barbara Tolley, but insists on being called "B.J." even though her middle name is Arabella.
 * Also subverted with BKR, the villain of Rod Albright Alien Adventures. This is his whole name, and none of the other aliens seems to think it odd.
 * Robert Heinlein's The Number of the Beast. One of the characters is named D.T. ("Deety") Burroughs. She goes by her initials because she's embarrassed by her real first and middle names: Dejah Thoris (her father Jacob was a fan of the John Carter of Mars books).
 * Y.T. in Snow Crash, though she does explain what it stands for once, to an incidental character who heard it as "Whitey" and is considering taking offense.
 * TS Garp in The World According to Garp. He's officially named for his father--except his mother never knew his father's first name, only his army rank. "TS" stands for "Technical Sergeant." Everyone just calls him Garp.
 * In The Film of the Book, teen Garp tries to convince his girlfriend that it stands for "Terribly Sexy."
 * Members of the V.F.D. in A Series of Unfortunate Events are referred to only by their initials to preserve anonymity. This results in complications, as The Unauthorized Autobiography demonstrates during a transcript of a meeting, and in The Penultimate Peril when a total of three characters have the legitimate and undisguised initials.

Live Action TV

 * JT (James Tiberius) Yorke from Degrassi the Next Generation.
 * Degrassi also has KC Guthrie. Unlike JT, KC's full name has never been revealed, but Fanon is that it's Kirk Cameron Guthrie.
 * DJ (Donna Jo) Margaret Tanner from Full House.
 * DJ (David Jacob) Connor from Roseanne.
 * GOB (George Oscar Bluth) from Arrested Development.
 * JJ (Jennifer Jureau) from Criminal Minds.
 * B. J. Hunnicut from Mash. Lampshaded in an episode where Hawkeye tries to learn what "B. J." stands for, only to find "B. J." on all of his friend's military records. When Hawkeye asks who would name their kid "B. J.", B. J. answers "My mother, Bea and my father, Jay!"
 * C.C. Babcock on The Nanny, as is much of her family (sister D.D., mother B.B.). Her full name, revealed in the finale, is
 * JD (John Dorian) from Scrubs
 * DJ from The Latest Buzz
 * B.J. and the Bear. The title character B.J. (Billy Joe) McKay.
 * B.A. Baracus on The A-Team
 * JJ (James Jr.) Evans from Good Times
 * A.J. (Andrew Jackson) Simon from Simon and Simon.
 * In The West Wing, Claudia Jean Cregg is nearly always referred to as CJ Cregg. The rare exceptions are usually either formal occasions or wry comments.
 * JK (pronounced Jake) from Kamen Rider Fourze
 * Power Rangers has two: T.J. Johnson from Power Rangers Turbo and Power Rangers in Space, and RJ from Power Rangers Jungle Fury. Their full names are Theodore Jay Jarvis Johnson (as revealed in the "Forever Red" team-up episode), and Robert James (revealed by a children's book and never used on the show), respectively.
 * Melanie and Lindsay's (and Michael's, since he's the biological dad) daughter, J. R. (Jenny Rebecca) on Queer As Folk.
 * D.B.Russell on CSI. Kind of justified, since his first name is Diebenkorn.

Music

 * ZP Theart, former lead singer of Dragon Force. Rumour has it that "ZP" is legally his first name, though.
 * Starflyer 59. On The Fashion Focus, Leave Here a Stranger, and every album since then, the various band members have just been credited by their first initial and last name.
 * KD Lang. Although in some lyrics to her songs she refers to her actual first name (ex. "The Mind of Love": "Where is your head, Kathryn?")
 * Soul Coughing's M. Doughty, although once the band broke up, he started releasing solo albums as Mike Doughty.

Newspaper Comics

 * PJ from The Family Circus
 * R.J in Over the Hedge
 * B.D. from Doonesbury.

Theater

 * Archibald MacLeish's modern take on the story of Job: Job (and the play) is called J.B.

Video Games

 * Deus Ex has J.C. Denton, which is actually a code name. This is a hold over from the development when it was an ambiguous name to support the option for a female PC (it was latter dropped to lack of space for the voice acting for her and the he/she brother/sister lines) and as a Significant Monogram.
 * Halo 3 ODST has the Rookie; the only traces of any kind of name he has are the initials J.D.

Western Animation

 * Top Cat was always known as TC
 * B.P. Richfield from Dinosaurs. Considering the way names are given in the DinosVerse, BP might be his actual name.
 * TJ (Theodore Jasper) Detweiler from Recess. A movie stated that he got his name because apparently, a young kindergarten kid named Gus Griswald said he'd call him TJ because those were his favorite letters in the alphabet.
 * In Arthur, the protagonist's little sister D.W. (short for Dora Winifred). Lampshaded in an episode where she gets lost in the White House and her parents are describing her to security - one of the agents says incredulously, 'You call your daughter by her initials?' (or words to that effect).
 * J.T. Marsh from Exo Squad.
 * A.J. from The Fairly Odd Parents.

Web Comics and Web Original

 * PeeJee from Something Positive. Well, she's Penelope-Jennifer, but no one calls her so.
 * T.C. "The Playah" from El Goonish Shive.

Real Life - Writers

 * Male
 * Professor M.A.R. (Muhammad Abd-al-Rahman) Barker, creator of the Empire of the Petal Throne/Tekumel Tabletop RPG setting.
 * J. F. (Jesse Franklin) Bone
 * GK Chesterton
 * EE Cummings
 * TS Eliot
 * J. B. S. Haldane
 * CS Lewis
 * HP Lovecraft
 * JRR Tolkien
 * A. E. (Alfred Elton) van Vogt
 * PG Wodehouse
 * AA Milne
 * D. J. (Donald James) MacHale, director and author of The Pendragon Adventure Series and The Morpheus Road Trilogy.
 * HG Wells
 * E. E. "Doc" Smith of Lensman fame.
 * S. W. Erdnase, author of The Expert at the Card Table. Bonus points for Erndase's true identity remaining a mystery to this day.
 * Female writers (who may be using initials to hide their sex)
 * J. M. Dillard - really Jeanne Dillard Kalogridis
 * TV writer D. C. Fontana
 * "K. C. Hunter" from the Star Trek Deep Space Nine episode where Sisko hallucinates that he's a sci-fi writer in The Fifties is an homage.
 * P. C. Hodgell, author of the Chronicles of the Kencyrath
 * C. L. (Catherine Lucille) Moore
 * JK Rowling - she indeed used initials to hide her sex, she doesn't actually have a middle name.
 * P. L. Travers (Mary Poppins series) - really Helen Lyndon Goff
 * Historian C. V. (Cicely Veronica) Wedgwood
 * CJ Cherryh, who also added a gratuitous 'h' to her last name to make it seem less feminine.
 * KA Applegate, author of Animorphs, Everworld and Remnants

Real Life - Other Famous Individuals

 * PT Barnum
 * JM J Bullock, who is only JM J because Jim J Bullock was already registered with the Screen Actors Guild
 * TK Carter
 * JP Morgan
 * CCH Pounder
 * New York Yankees pitcher C.C Sabathia.
 * M Night Shyamalan
 * JEB Stuart
 * OJ Simpson
 * SE Cupp
 * W.C. Fields