So. The general concept I'm going with here is a relatively normal woman unintentionally has a demon bound to her through some accident, incident or outside force/influence with this event preferably doubling as the call to adventure.

Summoner needs the demon to achieve her goals effectively because she's blind and because the demon doesn't have the same aversion to hurting people that the summoner does (like a good cap/bad cop duo) meanwhile the demon has an incentive to keep the summoner alive because of the shared HP and its inability get further than 100ft from her.

Blessed background would be explained by Shelyn or another god who sees a silver lining here: A chance to see if a relatively pure soul could redeem something as corrupt as a demon when the two are forced to cooperate.

Here's what I have so far.

I am Melody Brightwater. I would like to say that I am a perfectly normal woman, but I suppose that would not be entirely accurate. For staters, I am an aasmiar and I am blind. I was found in the small village of Greybud as an infant, laying in a basket left upon the steps to a Temple of Shelyn. If you have heard many stories, then it probably comes as little surprise to you that the temple took me in and raised me as an acolyte.

It was a quiet life, an uneventful and completely peaceful existence, but one I found contentment in.

Suggestions?
The main thing I'm stuck on is what sort of incident may have caused Melody to have a demon bound to her, how to go from the what I've written above to "Oh, I'm stuck cooperating with a demon now."