I can see how the fact its a negative energy rather than a straight up brainwashing would change the technical aspects of the effect for most characters. However its still a mental effect and Koishi's immunity is an absolute thing. She can't be influenced because, for all intents and purposes, there's almost nothing TO influence. A good chunk of her mind is mind is missing and, while she can feel great emotional intensity, it would need to be one of the four cardinal emotions (joy, sorrow, fear, anger.) She's actually completely incapable of feeling hate, hope, long-term desire, romantic love, and many others. Because she's incapable of most long term complex emotions. Now that's not saying she can't get mad. As I said she can feel anger and is quite capable of killing. She's done it before. But this is because of something that is actively happening to her or someone under her care. And even then all she'll remember is "This person is not very nice. I should avoid them because I don't want to hurt them."

The touhou power scale is strange like that. Characters are "absolutes" in whatever area those powers revolve in.

Interestingly enough, however she does have a single lingering semi-regret but that's against herself. For damaging her mind reading ability in the first place. However that's still very much "Oh god what was I thinking? How could I have hurt my sister like tha- Oh! A shiny! 8D"

I suppose the point stands that it is an event and things. I probably wont join then and just assume Koishi went and left without making an impact, or probably even being noticed, and that's fine too, though unfortunate. I'll poke you if anything comes to mind.

(on an unrelated note that particular shot of azula you used for the icon gives me chills for some reason.)