"Uh, the reason people look like their parents, among other results, is a sort of chemical set of instructions called a genome in every cell of the body. Except the red blood cells. Daeva do not have genes, by default, but ex-human daeva at least have genomes that used to be theirs. Indestructibility applies to whatever seems to us to be part of our body plan and it's a little challenging to add things to it for extended periods of time, and if we slip whatever is newly destructible doesn't last very long because we're not doing biology in a way that supports things that can't cheat at staying alive with magical support. But I think if he really really wanted to he could get genes into at least some of his cells and since there does seem to be a wizard gene that might do the trick. ...This also implies that with sufficiently sophisticated gene therapy arbitrary humans could become wizards."