Not an OC, but a character from one of my favorite animes that I decided to rework a little (very little) for fan stuff.

Cursed by her old mentor, a strict magician, she now transforms into a mouse whenever she tries to use magic. Once a talented and promising sorceress, she’s now chosen the way of the sword. Ihrie shows many scars that would identify her as an experience warrior, if not for the fact she got most of them while on mouse form. In fact, her most prominent one (a scar across her left cheek) was made by a cat. She gets easily startled. Now she often needs to rely on Fam, which she hates. Ihrie seeks a way to lift her curse permanently (she stocks on antidotes that transform her back into a human but that's a really uncomfortable way to deal with the curse) and to get revenge on her old mentor.