Good series with a rushed but mostly satisfying ending. I am still a little confused about Oga being able to see the wings. It might have something to do with caring about the person? Like, obviously Sayuri cares about Mikoto, but maybe since Oga is so kind that she has a general level of sincere caring for other people that she can already see Mikoto's wings from the start. It could have something to do with being a lesbian I guess, but Kusanagi couldn't see Mikoto's wings. So it must be something to do with the relationship to the character.
The other thing I don't get is that Kusanagi saw Sayuri as an angel during their school days. That supports the idea that seeing the wings is down to caring about the character. It kind of creates a new problem, though, because what are the wings caused by? Nobody sees Sayuri as an angel at any other point in the story, so it's not a permanent thing. Was seeing Sayuri as an angel just a kind of metaphorical thing due to falling in love or whatever? Because that seems out of place in a story where Mikoto's wings are real and physical. I basically guessed the ending pretty early on and I always assumed the wings are due to having a proximity to death or being in a borderline state like a coma. But Sayuri having the wings in the past complicates that explanation.
The wings seem to be inconsistent between Sayuri and Mikoto. Mikoto is aware of her wings, while Sayuri appeared to not notice them. That would seem to indicate that Kusanagi imagined seeing Sayuri as an angel due to love. Again, though, why include a detail like that in the story when Mikoto really did appear as an angel? If Sayuri really did have the wings once, then we need a new explanation for the wings. It could be an emotional thing like experiencing despair or depression. I don't know, I'm not having any great ideas.