"Why do all gender-bent characters have names ending in -a, that's such a fanfic trope" buddy, the "girl names end in -a" trope is so old that JRR Tolkien invented a Hobbitish dialect of Westron in which "-a" is a masculine name affix, then turned around and "localised" those names to end in "-o" in the published text (e.g., Bilba > Bilbo, Maura > Frodo, etc.) so they wouldn't sound feminine to Anglophone readers.
Also isn’t that trope just derived from the fact that a lot of the Romance languages are gendered and that has influenced European linguistics and naming conventions to sound like that?
In part, yes, though the majority of personal names in most English-speaking cultures are not descended from Latin; this is one of those situations where you need to be cautious about leaping from "this influenced the situation" to "this is The Cause of the situation".
I was curious so I looked this up.
Apparently in Old English, -a was a typically masculine suffix. (Which may be how Hobbitish Westron wound up that way?) But I think that's unusual in Proto-Indo-European, which had -a as a typically feminine suffix. Which would explain why it's common and not just from Latin names.
It does raise the question of how it became a masculine ending in Old English, though.