So, idk why but I am really curious about this. I don't recall who said it, but someone in the game says that Athena made it so women can't be the master, because they're too kind.

Does this apply only to the biological sex? What if someone was trans, and began their transition after getting the will? Would they loose it or keep it?

You are trans before you start transitioning. The question is more on wether it only accounts for the gender you are born in, or if it may recognize your gender's "soul". That's assuming souls aren't genderless and based on the notion that they can be seen by someone like Robert.