The moment Reimu bit into the yukkuri food she let out a sudden scream. That sudden outburst scared Marisa.
"What's wrong Reimu? The food doesn't suit Reimu's taste!?" "YUWAAAAAAAA!!!"
Reimu didn't hear Marisa's words. Reimu continued crying, but it wasn't long before something started happening with Reimu.
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"YUGUYUGU!!!" "What's wrong Reimu!? Did you put poison in Reimu's food!?" "I didn't put in any poison.... But its something different." "Does that mean you put in poison!?" "It's not poison... but it looks like their body will break down!!" "Does that mean..."
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And that's the sort of exchange that went in on her imagination. Even though there's no effect on Marisa, there has to be some sort of effect from the poison on Reimu, right?