Although this seems highly unlikely, I force myself to grade my slim hope of Irene's survival, since she was my favorite. I actually think the demon could be Irene. Yep..
Remember when Irene told Raphaela that she might stand a chance against her if she had both her arms intact? Raphaela, as we all know, has power equal to a number one (Luciela) who became an abyssal one. If Irene is to have a chance against Raphaela, does that not imply that she has the power of a number one? Irene knew, right away, that Raphaela, who has her yoki sealed, was not just a simple number 5 so i believe her judgment to be pretty accurate; if she surmised that she might stand a chance against Raphaela in good condition, I choose to believe her. Irene might have always had the strength of a number one but Teresa's existence was her obstacle to claim the top spot. Plus all the years of her hermitage, she might have trained hard and grow to be a lot stronger than she used to be. If we accept all these for the moment (regardless of the lack of strong evidence), we can agree that an awakened Irene (who has comparable strength to Raphaela and Luciela) can effectively be a threat to Isley. We do not have clear evidence that Irene was killed by Raphaela so her awakening and escape from Raphaela is probable. Isley marched south, battled and killed Luciela and eventually run into the awakened Irene, who poised to be a threat.
There are holes in my theory though, like if it was Awakened Irene, then why did no one notice her awakening? The presence of another near-abyssal level would be something of note.
And another weird thing is; why would Isley be putting out so much youki ..