I reread the whole series again, so the Red Potion is needed because the Singularity (the one in Karinz, the snowy place of invasion from ODERS/undead battlefield) has used the mysterious Red Potion to turn back ODERS into a human again. [i.e. Marie, granddaughter of (forgot her name, that old woman who turned into a DR)]
ODERS were created by those Singularities, so of course, they have a way to turn them back to human, because they know the process.
That's why MC and the gang is in that place right now, to properly learn of a way through the knowledge of PENS~ of books and research materials, to learn of how to acquire the Red Potion. But of course, PENS- have other things in mind, and that's one of it: to analyze the full potential of MC's prowess.
Now, the main answer. To whom they'll use this Red Potion? iirc, she's the bodyguard of the Princess of Stormheim (MC's childhood friend) who got infected by ODERS blood.