So I had a new thought regarding Touhou
Specifically, Marisa. One thing that is basically known is that the characters from this series are supposed to have particular abilities mapped out to them, either that they were born with or they acquired at some point. I figure that this is not any different for Marisa, either. However, as far as I can tell, no special abilities are actually stated for her, just her magic (which she taught herself first, then Mima taught her later), and an artifact that she uses to boost her power enough to make her actually strong enough to hit well above her weight class (the Mini-Hakkero, a magical furnace that is able to amplify the magical power fed into it).
So then, what power would Marisa have? It's actually fairly commonly used in fiction, fan or otherwise. In this case, I'm talking about being able to near-instantly copy magic that she sees used; an intuitive ability to understand the magic that is cast around her and immediately be able to use it. It explains why, after seeing Yuuka use her Master Spark laser, Marisa was able to use it as well, not to mention getting access to Patchouli's Non-Directional Laser as well. It also explains Marisa's frustration with Reimu's power, since it's not magic but spiritual power, and that's not something Marisa can copy. In fact, I'd go so far as to say that Marisa is a slow book learner, but if she's in the presence of someone casting magic, so long as she's paying attention, she will then be able to use it. The only reason she didn't steal more of Patchouli's spells is because Patchouli is foremost an Alchemist, and such Alchemy spells would generally require reagents; such things would be cumbersome to carry around, and many might be harmful or outright deadly to handle as a human (which Patchouli hasn't been for many years, being a Youkai Magician now).
Thus, it makes sense, and why it sometimes seems as though Marisa's magical repertoire is so haphazard, compared to some that might be more uniform. And now I have more to work with in case I ever get around to writing any of these myriad stories I've hinted at over the years. And yes, I still think about them from time to time. I just don't have the creative spark to actually do it, like, ever. Well, that or it's just my own inherent laziness...