Spirits can regenerate health out of combat (not all)

So i made a new rt class and ive gotten Union.

I cast it enter battle , took a few hits that got reduced to 0 by Union, Union takes damage for that all normal.

But then i killed the last monster and went out of combat at which point Union started to regenerate health back to 100%.

I never knew some spirits can regenerate health outside of life stealing with their attacks or using my own skills to heal them. Wonder what other spirits regen health

Eddited: Pain can also regenerate health after combat. Im also assuming other rt spirits can do so as well

Every spirits