Marisa Kirisame is Thief of Light cus she is self-proclaimed rival of Reimu, she likes spotlight. Her hobby is stealing magical books and plagiatising danmaku (light bullets). And as human magician, she does not produce magic - she borrows it somewhere. And most her attacks are light based. And she is rather egocentric and full of confidence and bravado (at same time).

I have nothing to add onto this, this is a solid analysis and I’m also happy to be getting Touhou asks. Marisa is definitely Light, and she’s most definitely showcased as a literal thief. Does this mean that the moment she stops being Human and becomes a Magician Youkai will be when she has reached a critical mass of Light stolen?