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 names, and calling them something like Girl A on screen. This is usually not an instance of They Just Didn't Care, but is done for its comedy value.

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.

Tabletop Games

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