It doesn't help that their current opponent is one of the mentally weirder monsters in Undertale. The Vegetoid seems to have a sort of selective intelligence; it can speak a little, but it doesn't seem to be aware of this fact, and denies it every time it's spoken to.
It's smart enough to understand that a hand on the stomach can indicate hunger and that vegetable-shaped bullets resemble food, but not smart enough to realize that white bullets hurt people, and that its violent behavior can scare people away from touching ANY of its bullets, even the beneficial green ones.
Put all of this together, and it leaves Frisk in a dangerous position. The only way to placate the Vegetoid peacefully is to accept its "meal", but the thought of doing that brings up memories the poor kid doesn't want to relive. Especially not when their HP is low, and they don't yet know that they can self-resurrect.
Also, on a meta note, THAT RIBBON. I knew that ribbon was going to be trouble the moment I started adding it, and sure enough, I have nearly forgotten to put it in almost every panel where it appears. Heaven help me.
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