Interesting tidbit:
When Kirin brings Akira back to karaoke for confronting him about Karin, Akira starts acting weird and the blood from Ami's murder comes back to the window.
Mind you, when he's there with Karin there's no blood on the window, nor Sekai talking to him - just the Old One.
Just from these two, it would suggest that Sekai is related to the blood/corruption. Pareidolia is just there for both since he's inside of Akira's head. Further evidence of blood being related to corruption, and thus possibly Sekai is found on Paper city.
This could all be nothing and just Selebus choosing which scary thing he'll throw at the background, but it could also be something, idk.