Yuno is much an elf as Gauche,Marie,Klaus or even William Vangeance for that matter.
but what sets him apart is that he is the only elf with total control over his body unlike others who remember a life before this as elf.
But that doesn't mean that the reincarnation magic didnt work properly or completely like Rhya wondered, because it did.
the reason Yuno doesnt possess a different elfish soul or a elfish personality is that he doesnt have one to begin with. Yet he possesses a mark on his face signifying he is an elf. Now who is an elf but didn't have a life before this.
Simple. Licht and Tetia's son in the womb yet to be born
What about Asta then?.......Well our beloved muscular freak is just an anomaly with no mana.this is what makes him unique and also allows him the access to Anti-magic like no other.This is what makes him a formidable enemy