dent entity that is free to torment whoever she chooses.
Regarding Maya:
Fallen Angel theory -- Maya is a divine entity who willingly forsook her divinity to be with Akira. Support: The Altair and Vega story. To use the language of the recent update, she defied her Legend, yet continues to exist. Since this cannot be by the rules the gods play by, she is one who should not and can not exist. Yet she does.
Illusion theory -- Inspired by her name, this implies that Maya isn't real, and never has been. She's something that Akira willed into existence by sheer need, and his conviction was such that everyone else perceives her as well. As a divine host, Kaori pierces the illusion and does not realize she's there.
I lean more toward the Fallen Angel theory, because Kaori in this update made it clear that she knows that Maya exists, but that Maya shouldn't exist.
With Sekai, think "this town has two halves." This town is the world, and Sekai IS the world. The things about her that were deserving of Akira's love are all within Kaori now. The things about her that drive Akira's trauma are external from Kaori and wholly independent of her. they represent the two directions her life might have taken had it not been cut short.
There's a lot to unpack here, and I'm still not done with the update (I've read spoilers about what happens in and after Sensei Quest, but stopped playing at that point because I was passing out tired, I'll get back to it after work).