Well, then Ami his schizo daughter/niece ghost as well xd
But I understand what you mean, considering the event in which Akira fingered Sana he was talking with Sekai right before this, but Sana didn't see anyone. YET, we know Sana can see supernatural shit, so it is likely Sekai is really just part of his broken mind.
I was about to write that event as evidence and you beat me to it. I think I still need to clarify a bit what I meant.
Sekai was a real person that existed, she is the one who abused Akira and likely made him become the same as her, then she died.
The Sekai that Akira keeps seeing are just hallucinations of his broken mind, he is so far gone that he is probably a schizophrenic as well.
It is already established that if the world is fucked and if there is something supernatural going on, everyone can see it. Maya and Ayane saw the world being distorted which means it's true. Even Yumi and Tsuneyo saw the world being reset before their mind was wiped which means that everyone can see if something supernatural is happening before their eyes.
But in the nurse's room, when Sekai was talking to Sensei, Sana couldn't see her, she just saw Akira talking to himself, and this is despite Sana also hearing things and having similiar problems as Akira does.
Ghost Sekai is just a part of a sick man's mind, and it makes sense if the gods can see her too because they can see inside Akira's mind as well. Maybe she was implanted there by one of the gods, maybe not, but she isn't real.
Regarding the Sekais that appear in front of others that are not Sensei, they are probably doppelgangers taking on the form of the person that can fuck the most with Akira, but SEKAI herself is long dead.