Here's my theory, with the latter part going into the crackpot realm. Ryou is the reincarnation of Okiku's past lover. Okiku on her death regretted that she wasn't able to stay with Ryou's past self, giving rise to great ghost 1. She also hated everyone who she thought stood by and let her be killed over the plate, giving rise to great ghost 3.
This gave Okiku a split personality with 2 ghosts in one 'body?', with ghost 1 being filled with desire, love and other positive emotions, and ghost 3 being filled with hatred, anger and other negative emotions. Ghost 1 is stronger than ghost 3 when it comes to control over the body, but ghost 3 can perceive ghost one, while the opposite isn't true. Probably explained by how we bottle up our negative emotions and don't notice them ourselves.
It's extremely unlikely that Okiku will ever kill Ryou, Ghost 3 can't keep control for long enough. I see it as being a fakeout, with something else having hurt Ryou.
Now for the weird jump in logic which probably will be wrong. I think that Meguru could be a partial reincarnation of Okiku. Since Okiku is 2 ghosts in 1, there's simply not enough room in the body for 2 ghosts. The rest of her, parts of both ghost 1 and 3 has reincarnated as Meguru. This is why Meguru and Okiku feel similar. It also would explain why ghost 3 appears in shadows and mirrors around Meguru, and her intense love for Ryou. It would mean that the friendly ghost in Ryou's past attacked meguru because she felt the malice in her, as otherwise she went really quickly from exorcise me now to attacking Ryou's friend.