Plot twist: as the author is the antagonist, Yona is the incarnation of a sensible editor, who tries balancing out the worst excesses of the author. So she hands the heroine the tools necessary for victory.
I'm guessing she left the knights because they tried set her up for failure, or because she got pregnant... I can't really tell what her age is, but based off of years of experience in guessing I'm gonna say she's... 26-ish...
I really hope she didn't leave due to harrassment.