The Sekai situation is very complicated. There is some evidence that Sekai was the one that had her hands on Akira's shoulders, and that is present throughout most of the game in various events. There are also multiple characters and narrators that say that Akira has the weight of the world (Sekai) on his shoulders. Then AmIOkay even calls Akira Sekai. The same "Huh?" that Yasu says to Sekai in "Shelter", that fdsasdf_p pointed out, can be seen on an early Sana event, where Yasu says that there is someone with Akira that he can't see, and looks to his side and says "Huh?". Yasu also says in "frankincense" that she can reveal the voice of the one that follows him, and if that is also Sekai (high likehood) then she's even more present. So, basically, Sekai can be the most present character in the story, we just couldn't see her/currently still lack confirmation it was her.
If everything I said turns out to be true, then it also adds complications for the "Sekai wars" (multiple sekais theory), since it would point out to many Sekais existing as of Chapter 4. If white dress Sekai can only haunt Akira and Ami, then how come a Sekai left them during that one cemetery scene saying she had stuff to do? If Sekai is the one following Akira, then how come he runs to Kaori for help against it and is helped by another Sekai inside of her? As always: