I highly recommend trying to get the good/true ending: bring each boss to the point of being able to kill them, then spare them until their health regenerates (don’t use the finisher), then fight them again until they’re in the same position - the second time around you’ll get an additional button prompt, beyond just “finish them”, to “spare them”.

Do it for each boss and you’ll unlock the true ending. Probably one of the best moments I’ve ever had in a video game.

If you’ve finished the game with age 20 it shouldn’t be much harder!