Sekai in the past and Sekai now seem like completely different people personality wise.
This is
the update that has me thinking, either:
1) perhaps the ghost Sekai that haunts Akira isn't a "natural product" of a dead person's spirit, but a curse that Akira subconsciously placed on himself,
2) or simply the voices in her head and/or the life she was in haven't gotten to her yet.
Option 1 has been discussed quite a lot, but mostly in the direction of simply deciding whether the free floating Sekai is real or only exists in Akira's head; since we can find more than one scene where Sekai could directly or indirectly interact with other characters, we presumed ghost Sekai is "real to everyone".
However, after revealing the human side of Sekai in this update, I can't help wondering about additional questions like "Why does a woman who supposedly only cared about Akira then Ami and probably nothing else when she was alive, become this evil sex nymph that we see today after her death?" Could it be that the ghost Sekai isn't a 100% representation of her original self, but a twisted conjured entity with her darkest traits emphasized and amplified?
Essentially, this is the application of some rather common ideas in media when it comes to interacting with dead spouses' spirits. Either their imagery is infested in a horrifying manner by alive spouses' negative emotions like resentment and guilt toward the dead ones, or you simply cannot summon a 100% representation of a dead one's original self anyway; you'd always summon
something that feels like her but not her.
Combining other information in this update, namely "how to become a god", "everything you can imagine is real" by Pareidolia and the eerie end of this happy event (Sekai's funeral photo as soon as she said "I don't think I'd be able to [leave you] if I tried"), it feels like the ghost Sekai is literally a curse Akira placed on her that results in a terrifyingly twisted dead girlfriend spirit who not only haunts Akira but also is capable of free roaming and pestering others.
Option 2 on the other hand is a lot more straightforward; her descending into madness at that point was far from completion.