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.

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 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.

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.