EDIT: the thing with the healing effect is that it increases the effect of DIRECT heals from the character, but only the character itself gets a regen increase.
What this means is that if Amaterasu had it, she'd recover more HP than the others, but if Brynhildr had it, she'd heal everyone by more. It's more or less a pseudo ascension...
considering i got a 1%+ and a 3%++ on Sol (by sheer light drop rate luck), her heal with her buffs jumps from 2700 to 2772, but only she would gets 385 regen HP per turn instead of 375 like everyone else
this shows it off:
https://www.youtube.com/watch?v=-WOvpDyBpXQ