Where did Dark and Krad come from? I've been rereading and rewatching DNAngel, and on thing that I can't remember is where the two of the came from. Did they just appear one day, or did someone create them?
Dark and Krad are two halves of the Hikari work "The Black Wings.".

As for how and why they come to manifest, it was never elaborated upon in the manga or anime.