All characters take less damage when they have less health. Check it for yourself by doing Fulgore’s heavy DP on a dummy with full health, then again on a dummy with 20% life. This is a reason why your combo at the end of the match doesn’t kill very often.
I think damage is decreased a little bit (5%) when you have half a life bar remaining, then more at like a quarter of a life bar (10%), then even more (25%) when you’re in the DANGER ZONE. To be clear, if damage is reduced by 10%, then a combo that normally shows “10%” will instead now do 9%, not zero.
“Taking less damage” at low health is pretty much the same as “having more hit points” at low health… So I divided up the life bar into segments. The first 90 hit points are normal, the next 45 are slightly stronger (+5%), then the next 30 are stronger (+10%), then the last 15 are the strongest (+25%). That’s what I mean by “effective hit points”: not the number of HP you have, but the number of HP your opponent has to inflict on you because of the damage reduction.
ARIA has smaller life bars, meaning she gets to her damage reduction faster than the other characters. But it turns out that it doesn’t matter, since in the end she has the same amount of “effective hit points” as everybody else.