thank you @seamiint for the comment!
achlys, adonis, and natt are kind of tied for my current favorites, so i'm going to answer for all three of them.
if achlys found out he was an oc, she would have a severe existential crisis immediately. he would not deal with that information well at all. finding out that she is one of my favorites would not help. he wouldn't understand why i would make him go through so much if she was one of my favorites.
on one hand, finding out that he's a favorite would just boost adonis' already overly inflated ego lol. on the other hand, he would also freak out if he found out he was an oc. he would be extremely angry and vengeful towards me if he found out i was behind all he's been through. not to mention that learning he is a fictional character who is controlled by the whims of another would definitely scare him and make him feel helpless and vulnerable
natt would probably react the best. they wouldn't know how to feel at first about being a fictional character, and like the others they wouldn't understand why they've had to go through the stuff i've put them through. but i think they would be overjoyed at being one of their creator's favorites, and they would feel comfort in knowing that at least i love them.