Ok so this can be summarized to one thing: Only those characters in-game have access to the power of passive regeneration. Nor Kai, Nor Merlot, nor any other npc have the level of regeneration those characters have.
The Player, for example, can withstand life-threatening injuries like losing limbs with ease, and we don’t know what King Calvus was referring to this yet in this phrase, but I think it’s safe to assume that the Player is near immortal