(SPOILERS) Okay. So this is something im sure you are all familiar with. DETERMINATION, the red soul. So, lets think about what it has done to everything first, and With 2 Sections, I will describe what happens with each species.
We all know that Frisk/Chara's soul is red, the theory is that it means "Determination." But the effects on the other souls may not matter. It may just make them a tiny bit stronger, but not much. It wouldn't amalgamate them, seeing as they have a much more complex physical form. But, then again, an overdose may amalgamate them with a monster.
Now we're getting into deeper stuff. The DETERMINATION was originally supposed to help a monster soul be used as a means of breaking the barrier, but it backfired, Instead, the tested Monster's bodies "Melted" and they changed shape, fusing with the others. This can also be seen in Undyne the Undying's battle, as once you kill her she melts, Like the amalgams.
How it was discovered:
The theory that has been going around was that Alphys did not discover Determination, But W. D. Gaster did. This is not Canon so I will end that here.
- Determination is extremely deadly to monsters.
- It is Theorized to be red, But this is not confirmed by Toby Fox.
- Determination is found in every Human soul.
- Never found in a monster soul
AU's Determination is in:
Literally All of them.