Hey. I was the anon who asked for help on my heartless character. I just saw your respond to it about some European folklores?
Yeah, I´ve heard of russian, rumanian and also hungarian folklores containing this kind of character - sadly they play the villain part. Never hear of these characters in western european folklores so far. It seems to be a more eastern thing.
Usually this heartless character is someone very strong (physically) has some other impressive features- like size (being a giant), super sense of smell (they spot the human hero in their hiding places). They are also described as nonhumans although they have a human appearance. Sometimes they eat human flesh.
Now that I think of it - there is also the Golem as an artificial being, needing no heart, and there are the Ushebti (little artificial creatures in the same manner being servants for the pharao in the afterlife - ancient egypt) In newer stories there is Howl´s moving castle (which you should also check)
Having no heart (in european folklore signs of it would be: cruelty, emotional coldness, loss of joy and friends, being physically invincible, those superpowers I described earlier, being able to use magic) is not opposed to being alive.
In reality there are illnesses destroying hearts and under rare conditions there is the possibility of using pumping- devices doing the work of a heart. Earlier surgeons took the heart out to make room for the motor and the pump, nowadays, they leave the organ in the thorax (cause those devices are getting smaller and thinner) So you see even in reality it is possible to survive without a heart for a period of time, and under right circumstances.
You have to decide, what importance a heart has, in your story-universe - and then think of the consequences.
Is it the part where the soul has its roots?
Is it where emotions dwell?
Is it making a human a human?
Or is it a cruel thing to have to obey to emotions?
Would you make a deal - to trade your heart to be physically invincible?
You have to get into the head of your heartless character - why did he/she lose his/her heart? Or was it a trade? Did it happen on purpose or was it a force from outside - like an accident, fate or a spell?
The reader will understand that it is an essential part of a functioning man/living man/woman - the importance of this organ is inherent. Everyone understands that. The meaning of a heart is only for you different. You have to clarify it for yourself first.
If you want to have specific folklore to refer to, I could do some digging, but it´ll take a little while since I´ve heard those stories as a child, not read them. So if you need more- write me.
I hope this was of any help! :)