User blog comment:B214/Question and Answers 6/@comment-99.235.66.221-20161130072837/@comment-10330822-20161213103823

Usually, its a genetic thing. Like how Le Fey can use magic because she is a descendant of Morgan Le Fey, and Georg can use it because he is a descendant of Faust. However, its never stated you *need* to be a descendant. In fact, I view it kind of like Harry Potter, there are "Muggle-borns", just random magic-using people who show up, and bloodlines of previous wizards.

Otherwise, I think you need to be reincarnated as something else. Like how Issei gained the ability to use magic by being reincarnated as a devil.