I have a feeling that Diabel is an Akemi Homura type of situation. A background story that is mention multiple times is the love story between lovers whose families are enemies. Diabel also mentions multiple times about making different choices this time around.
What if Erez and Ayana were actually romantically involved in one life time and Diabel was fine with that since Ayana was happy, but tragedy struck and Ayana was horrifically killed? Somehow, Diabel was able to regress on his own to repeat the timeline to try to save Ayana but after many failed redos, he finally made a last ditch effort deal with a devil.
It also feels like he's purposefully trying to keep distance from Ayana romantically despite being so obviously obsessed and in love with her, so maybe he sold his ability to receive love? Which he would've been fine with since he's resolved to be her dedicated knight only + he doesn't think she would've ever reciprocated his love (i weep)
So basically, my theory so far is that he's a blended mix of Hak (Akatsuki no Yona) and Akemi Homura (Medoka Magica) lol