^
I tried looking for mote info on the movie and apparently it’s based on a story by a late Edo period performed. Its name is Shinkei Kasanegafuchi.
My sources are pretty limited since I can only look for answers in english speaking sites, but I ended up finding this page about Kasane on Yokai.com.
It says that ”she is often held up with Oiwa and Okiku as a prime example of the grudge-bearing Japanese woman ghost”. So maybe instead of Otsuyu being the third ghost, it’s Kasane.
And the description of this ghost matches the one that has been lurking around.
Now, while I could draw a parallel between Otsuyu’s story and Ryouyo’s current situation, I have no idea how this ghost could be related to her and for what reason she’s around her all the time.
I thought that maybe Otsuyu would be jealous that someone would possibly have a mutual love with a ghost, something that was denied from her (ghost) in her story. This wouldn’t explain why she’s been there since Ryou was a teen (and maybe earlier), but oh well.
The only reason I can think of for the ghost to be “haunting” the MC is the fact that Ryou is most likely the reincarnation of a woman from the Edo (probably the one related to Okiku’s tale).
Lastly, the bridge Oiwa saw Ryou dead and the one we see during the flashback where Okiku is meeting the Ryou-looking faceless girl could be the same (????).