Dark aeons are there just for the fighting, you won't aquire them, but they tend to drop armor with Ribbon on it, which stops EVERY status effect in the game (small excepts). Your HP can at maximum reach 99999, but to get it over 9999 you need the Break HP Limit ability on your armor. Your best bet is to win this from someone, like dark aeons

There are more dark aeons waiting for you, and dark valefor is the easiest of them. And besides, your HP isn't neccesarily the most important stat, strenght should always be maxed out (at 255). Your weapon can do 99999 damage with Break Damage Limit on it, in case you wondered. Hope that helps