Ramming two skills on his list together because it's thematically appropriate for Josh to get this as his final skill! Josh is now functionally immortal; he won't age (unless he decides to, which he will to get himself to twenty-six or so -- that's another skill, though). More importantly, he'll automatically regenerate even the most heinous wounds, so long as he isn't decapitated, his spine isn't crushed to powder, and he has MP left. (This does apply to even normally fatal wounds, as in Billy the Kid's case.) The less MP he has, the less he'll regenerate, so it's totally possible to run him out of strength in Aather (not really as true in canon); in general, though, if he has any strength left in his aura, he'll cling to life. (If he were going to be participating in games, he'd be able to die in those, obviously, but that's not really an issue at this point.)

EDIT: I FORGOT THIS. He can also use this skill to suppress his aura now that he has his Aether magic. This means his heartbeat and breathing stops and his magic is no longer detectable; for most intents and purposes, he's neither alive nor dead. He can still use magic, but it's much harder to detect.