I read the walkthough here (in japanese, but google translate works)
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in short after you kill the red armor in demon pool's abyss, you still need to unlock another three area and kill the red armor there
To unlock true end you need to do three sub quest, get mirror from suzu, fight the fox before castle final fight (find tools, find a woman, find herb)
P.S. that took me a while to find the herb, the location is battlefield's nameless hill teleport shrine -> go west -> go down twice -> go west -> go up and west -> go down and east