I wonder whether you could do nostalgia for earlier in the same game. For example something Pokemon-ish where if you screw up your monster/character/whatever dies permanently, but there is a reminder of them left in the game. Maybe you'll encounter a weaker ghost version of them that may help you, or you need to collect the pieces of their soul and lay them to rest. And maybe after each encounter with them the likelihood of the next encounter decreases, making them unexpected and treasured.