Okay, so this is a niche I've spent way too much time in. Honestly, you'd think the half-angel, half-demon thing would be overdone, but it's weirdly hard to find a good execution. Most just use it as a shortcut for 'broody and hot' without really digging into the conflict.
My absolute top rec is the 'Fallen' series by Lauren Kate. The protagonist, Daniel, isn't explicitly labeled that way in the first book if I recall, but the whole reincarnated soul, cursed love, and warring sides thing heavily plays into that dichotomy. The internal war is the main driver. For something more urban fantasy with a strong romance subplot, the 'Guild Hunter' series by Nalini Singh has Raphael—he's an archangel, but the cold, powerful, sometimes brutal edge to him gives off that 'demon' vibe in the balance. It's less literal hybrid, more about embodying both extremes.
I bounced hard off stuff like 'Hush, Hush' though. Felt too much like the supernatural elements were just set dressing for a problematic YA dynamic. The real gems make the duality matter for the relationship's obstacles, not just the hero's aesthetic.