Well, I kinda understand your idea but this is simply not how it works, and it would kinda throw a big middle finger to the consistency of the events to come. First, you don't just casually stroll around and find a low level demon. Without any prior experience, it's pretty much stumbling while blindfolded on ice, hoping to find some kind of life form. And you can't really know how powerful it is, since demons don't really have any ways to respond, and you can't really gauge it's power. And if it does respond, you know you majorly fucked up. Spirits don't usually do benevolent things unless it is good for them in the long run.

That is an interesting idea that you have though, and I could see it working. In fact, it can fit in very well in the main story. But as I said, your starting state has to be, at the very least, unaltered. Seeking lost and forgotten knowledge will be the first major story arc for the mages. There is nothing wrong with your character losing said humanity, she just has to start with said humanity.