So there’s an interesting Reddit thread on who/what corrupted monk is. Highly recommend finding it as it’s extremely well written, but the gist is that she’s likely Okami (the Mask she wears is related to a kind of older Japanese theater/play, and the masks the other Okami women in Fountainhead wear are also associated with the same theater. They even perform dances associated with it), and the fact that her memory calls her Priestess Yoa is likely a reference to the legend of Yoa Bikuni (literally meaning “800 year old priestess/nun”). Yoa Bikuni was a priestess that ate the flesh of ningyo, which is a sort of Japanese mermaid, put simply. The flesh of the ningyo granted her immortality. Meanwhile, Corrupted Monk guards Fountainhead. Which houses the giant carp. Which can be considered a ningyo. On top of that, the Rejuvenating Waters come from Fountainhead, and eventually end up in the area where we first fight Guardian Ape (as evidenced by the carp showing up there when you kill it). The Guardian Ape is also clearly corrupted as evidenced by the same centipede as our Yoa (something else that’s interesting is that insects, arachnids, and stagnant water all mostly represent corruption in Shinto). So it’s possibly Corrupted Monk are the flesh of a previous great carp and achieved immortality through infestation because of it. Very interesting stuff!