Actual story spoilers up until part 2 prologue (or Moonlight/Lostroom) Don't they use it as the explanation for why the underworld fever does not effect him, which is after Mash looses Galahad
Ritsuka is clearly a creation from mash's own want for someone to be with her, and he/she is a creation of her therefore they inherited the ability; is my favorite retard theory combined