[quote=KabenSaal]
Would it be possible for Emily to, over time, become a Keeper in the Underworld? And then be the Under-Keeper?
[/quote]

Maybe Keeper over a small swathe of land down there, but not the entire underworld. We're talking about an entire realm, with millions of demons, many of whom are so powerful that I'm not even letting Shaige summon them as anything less than level five or six creatures.