[quote=@Archmage MC]
So, for kindred spirits, we're not supposed to kill their bodies and absorb their spirits? Or how does that work?
[/quote]

It depends on what you want to do with the spirits. If you chose a kindred spirit because you want its power, then you can absolute kill and absorb it. If you chose it because you wanted to play it as a character, that's when you would use as Friend Heart.