@amplified
Now that I re-read the Final Night and get to hear your theory of how and why she remembers him, I'm awestruck!!! I never realized the Rain arc could relate to the ending this way. It all makes sense that she remembers her past life because names carry memories.
I was wondering whether Sanko knows that his daughter is the reincarnation of Kochou. Now that I think more about it, when she was a newborn, he probably couldn't tell from the baby's face; he probably named his daughter after Kochou in her honor. But as she's growing up to resemble Kochou, he has probably begun to have a clue. That said, I think it's also possible that with his Akayashi sense, he might have known from her birth that she's the reincarnation of the girl he once knew.
So it's spring break in the story ending, that will give plenty of time for all of them to reintroduce themselves, catch up, go on dates in the Human Realm like they had resolved to do, and for the mother to get a grip of the fact that her daughter is going to pass up on college to become a bride to an Akayashi and take "moving out of town" to a new level haha. The mother seems pretty free-spirited herself so all of this should work out well! Oh, Shirogane will also have to swallow the fact that he's become an in-law to Menou, poor dude ha!
A question: do you think the girl is a sophomore or a senior? My over-immersed self wonders if she might decide to at least finish high school before resuming her past life in the Akayashi Realm haha
Seriously, though, I wish there were an epilogue that would tell how Shirogane lives now that he's become a complete water oni. What does he eat? Is he still nocturnal? What does he do for a living?
By the way, before coming across your theory of how and why she remembers him, my conclusion of this whole time was that it's her Akayashi half manifesting itself (as Akayashi don't forget what they've remembered).
Because she had been revived with the Art, she entered the reincarnation process as the soul of a not-Akayashi-and-yet-not-human being. That soul reincarnated in a half-Akayashi body, so there you go...
Plus, when Shirogane confessed his love in the Stopover arc, he said "Every time your name is called, remember me". I was thinking that even if the Reincarnation was named differently, when she is called by her old name - Kochou - it's her Akayashi elements that help her recollect the memories tied to that name. I was also thinking that this theory would answer the question the Reincarnation has about her lifespan: just like the ability to remember forever, she probably has a lifespan of an Akayashi too. So they live happily ever after, the one and only fairy tale I am willing to and can't be happier to embrace!~~~~~