Hi! I just came to the realization that my protagonist would fit the role of demisexuality perfectly. Honestly. I was trying to think of a sexual/romantic arc for him, but couldn't fathom his being concerned with the sexual side of things. I would really really appreciate it if you could give me some insight into a/greya/demisexuality, or how it feels to discover your own demisexuality. I want him almost as a guide for demisexuals who've never heard of demisexuality to answers. Thank you!
So, for me it was really fucking confusing, and it took me quite a while to work out that these feelings might be sexual attraction, and oh god what should I do about that. (It also turns out that 'you will know it when you feel it' wasn't true for me: I was really confused about what it was and whether it counted as attraction, and can still be confused, and I think that's a way that demis can (not necessarily will) experience sexual attraction differently from allosexual people. I believe metapianycist has written about similar experiences, and The Ace Theist has written about similarish grey experiences here.
Other people may be less confused and more like 'welp so this is a thing'. That's a thing which will very much depend on the individual person.
One of the main worries I had was how this would change the relationship I was in at the time and how to communicate it to my partner, so that might or might not be an issue for your protagonist, especially if he was already in a relationship with the understanding that he's asexual (which was the case for me). Or he might think he's allosexual but a late bloomer/whatever, in which case his thought process would be different.
Opening this up to input from other people; keep an eye on the notes. And here are some writing resources from anagnori.