Just a character in Book 2. Her name's Mage, Katarina's inner demon, having a hella fun time being trapped in the mindscape. 

Being trapped in here, in her mind, in her body, for too long, it wears me down every damn day.

I want control.

I want her to feel the pain I've felt for too long.

I want her to feel the longing to escape a prison she's locked away in for all eternity.

I've been trapped for too long. I want a turn in the living world.

I'll have my fun while it lasts.

If I can't escape my prison, then she can't either.