All the opponents_xxxx folders are interchangeable. Those are just the names I used to keep things organized. What you need to tweak is the opponent.type value in the opponent.properties file.
opponent.type=SELF this is an avatar that you can play as, but not against. (presumed male by the game, because I'm lazy)
opponent.type=MALE this is a regular player that you can play as or against, who will have male pronouns (This one is not used much in the code except in final undressing voice lines as far as I remember)
At least that's how it's SUPPOSED to work. There is a bug that prevents SELF players other than John Jamal and Jong from showing up in the avatar dropdown. I'll have a fix out with the next release. And since you found a bug, here is your bug bounty sneak peek of Gnome-Denise:
