[quote=@Vertigo]
So thinking of a character whose childhood beliefs ended up shaping their view/logic of the world at an early age, and therefore granting them "magic", but haven't settled on specifics yet. Like someone who believed in their imaginary friends so much they stayed "real" via magic, or someone who was told they were a demonspawn or something as an insult and they believed in it so much they grew horns and got some related magic, someone who believed themselves invisible to others so they can turn invisible, or something with seeing ghosts etc. etc.

Would something like that work in general?
[/quote]

Yes. Summoning is fine. Permanent physiological changes because of magic is a bit of a dangerous area (it's going to constantly be draining your life, attracting the supernatural). The demon thing would be more temporary and it happens when they use their magic as opposed to always having it. The rest of those work perfectly fine, though.