I've read some in-depth spoilers and they never mentioned that the two personalities merged. That doesn't mean I didn't happen though.
What I do find is strange is that I think the comic is going a different direction.
From what I recall, this isn't supposed to happen until after he takes away her senses. She is locked up, made blind, mute, and deaf, unless she is in his presence.
Her only escape is through dreams. Through dreams she learns the truth of her heritage through her father (she is a cambion and her father is a demon lord).
So, I have no idea what is going on. Unless those spoilers were all wrong or in the wrong order.
Definitely possible.
On a related note, that is probably why there has been delay in the release of the Tsuyokute anime. The last arc of the light novel (and subsequently, the manga) was absolute trash. It's been more than a year since it's announced release date. Japanese and English fans alike RAGED against it. Allegedly, the author was tired of it and wanted to move on and the mangaka just portrayed what happened in the original work.
Anyway, I'm not saying that the WN/LN is of this mangua is trash. I'm certainly open to a different ending arc, although I don't particularly like that the personality merge (I assume it is a merge) appears to have the OG Illiana as the dominant personality.
As another user said, I think this would be a good chance to get the MLs perspective on things. Flesh him out a bit more rather than relegate him to hot whacko.
Wasn’t og illiana and mc supposed to be two different pple and illiana is like a wondering spirit or something. But right now it seems she’s a reincarnation or is the mc misinterpreting her memories as her own