I was thinking that the plot is going to be one of the two when I saw the first episode (I didn't read the book):
1. Cirrus "saw" a different worldline while in a coma and "lived it", so when he woke up he felt that something was missing and decided to "recreate" the parallel world where the two of them were dating.
2. Cirrus liked Phu since high school (since that's when the two of them started dating according to him) so in his coma he "dreamt what could have been since high school" and when he woke up he had delirium and mistook his fantasy and reality (Since Cirrus asked himself "Which one is reality?"). But he knew many things, so it was either again the 1st one (since he said "If he doesn't know me here I'll start over), or, knowing how Thai dramas normalise red flags, Cirrus was a stalker? (Since Ai Jin asked Phu "You've never met his or you just didn't notice?" implying that Ai Jin often saw Cirrus near Phu. Plus when Cirrus asked himself "which one is reality" he was looking at a picture. A photo of Phu he took?, or maybe he photoshopped the two of them together? no idea). He stalked Phu since High School and became delusional that the two of them were dating? So after he woke up from the coma his delusion paired with delirium made him like that?
Again, I didn't read the book, this is what I had in theory was happening, glad to know I wasn't that far off after reading the spoilers.