Well, I guess you got where I am going with that girl, ancient mage decides to fuck with the beyond to escape certain death and goes around thinking: Heeeeey, if I take the traits of the demons and eventually lose myself in him, it works both ways! Finds himself a low level demon to 'Take control of him', making a spirit in his image. Is that spirit an ancient mage or just a sham? Who knows. Anyways, said cosmic entity is really eager to get some vengeance going and goes on to possess some mage more receptive to him. Maybe her parents tried to dable in the Beyond as they were in the run, they died. But the thing is that it isn't her seeking to open fuck with the beyond, its him trying to fuck with reality. So I'd argue that she isn't Over Power super high level as much as the guy on the other side sometimes assume control to do things. She doesn't work by knowledge, but by instinct as she after all had little in the way of training, and of course, using the focus of said ancient mage really helps the guy telling her what to do.

So, any ways I could make that fit in or should I just do something else?