Being the Emperor's hand, and you being Vader's prospective apprentice one day with the competency you demonstrated, Mara nevertheless deemed herself superior. She was your superior already in a way, possessing a considerably higher status than you were or could ever strive to be. Being the Emperor's hand was a privilege after all—you were just some apprentice.

Of course, Mara was delighted to shove that fact in your face as much as she could. Sometime she wished you could outlast Vader, maybe executing him with the Sith's 'only two' rule. And then she might kill you. It was all wishful thinking on her part. And she genuinely enjoyed the glares you'd both exchange.

"You know, it's a miracle they still keep you around." Mara announced as she sauntered swiftly into your chambers, as if she owned the space.

(She did this a lot. Just to gloat.)

"You're far too weak to even dream of being a Sith one day." Mara explained, giving you a look of disdain despite the smirk that was threatening to curl at her lips. 

And oh how she reveled in your scowls.