Girl A

Lots of films and television have unnamed characters who might need to give a line or two, and then disappear into the background. Because these need to be credited for the actors, they may have names like "Girl A" or "Man B".

That being said, this trope is about lampshading the fact that your characters don't have real-sounding names, and calling them something like Girl A on screen. They have to be enumerative names like letters or numbers, or else it's more likely to be Everyone Calls Him "Barkeep". This trope is usually not an instance of They Just Didn't Care, but instead is done for its comedy value.

In Japan, -ko is a common suffix for girls names. And names that sound like letters with this suffix are in fact real names, like Eiko, Aiko, Keiko, and Yuuko. If characters of these names grouped together, then it is an example of Girl A—or if the form is used to create not-names like Biiko or Shiiko (B and C).

This trope lies somewhere between One-Letter Name and Alphabetical Theme Naming, and is the laziest possible alternative to No Name Given.

Anime and Manga

 * Project A-ko, of course, has its main cast as A-ko, B-ko, C-ko, and D.
 * In Revolutionary Girl Utena, Nanami's Girl Posse consists of Aiko (I), Yuuko (U), and Keiko (K). Of course the usual deconstruction sets in here when Keiko's desires are explored in the second arc, as is her jealousy of Nanami.
 * The Shadow Play Girls are similarly named A-Ko through C-Ko in the television series, with D-Ko through F-Ko added in the movie.
 * On the first episode of Best Student Council during the Class Rep election, names for Eiko and Biiko pop up on screen, as if they were normal characters being introduced... and they are never heard from again.

Fan Works

 * Invoked and embraced by the title character of the DC Universe Online fic Employee 2814.

Film

 * Averted entirely in the Star Wars verse, where literally every creature that had a second of screen time got a name and a backstory in the Star Wars Expanded Universe.
 * While Xtracurricular, although inspired by Project A-ko, does not call its main characters by the same names, it does provide a Shout-Out to its source material's use of this trope in a secondary character called "Echo" (E-ko).
 * Played With in The Naked Gun and its sequels, where this sort of character doesn't even get this little of a name -- instead, the characters are listed in the credits by the lines that each of them said.

Live Action TV

 * M*A*S*H: Whenever "generic" nurses were part of a script -- walk-ons with at most a line or two -- they were generally given the names "Nurse Able", "Nurse Baker", or "Nurse Charlie", from the first letters of the military/ham radio phonetic alphabet as used during the Korean War period, making them "Nurse A", "Nurse B" and "Nurse C".  (And explaining why Nurse Able, for example, was rarely the same actress twice in a row.)  In later seasons this practice tapered off as they began giving these characters real names and turning them into part of the show's extended cast (like Kellye Nakahara, who played both Nurse Able and Nurse Baker before transitioning into the semi-regular Nurse Kellye).

Tabletop Games

 * Invoked by some GMs when players start asking for the names of too many NPCs that are not important to the plot.

Web Comics

 * We have an Erfworld fan comic celebrating the exploits of Bat 11. Bat 11 gets his canon screen time in Book 3, page 233.