My guess is that it’s directly related as to why our Demon Lord can’t resurrect his beloved hero.
“Similar but opposing power” doesn’t necessarily mean that the Will of the World has the opposite power to the Demon Lord.
It could just mean the Demon Lord has the power to ACTUALLY oppose the power of the Will, which makes sense when you consider how the Great Spirit said the Will has destroyed other safety mechanisms in the past.
Much like how the Great Spirit said that the Demon Lord’s current actions wouldn’t turn the guardians against him, the Will can probably avoid being attacked even when removing those safety mechanisms by simply staying within the rules.
My guess is that the Will is another entity like the Demon Lord who also has control over souls. But whereas the Demon Lord works through necromancy and the undead, the Will has more control over the living and is able to choose who gets reincarnated. The Will was able to drag John Doe’s spirit over so it would possess Ethan’s body as a split personality.
And I’m willing to bet that the final “boss” the Demon Lord will have to face before stopping the Will is going to be the reincarnated soul of his loved Hero.
Oooo...
The previous hero becoming the last boss before fighting the Will is a fun concept.
I have two contradicting thoughts about this. The people who were boosted by the Will had reason to fight the Demon Lord. The Hero wanted revenge, the Saintess and John Doe wanted to do it out of self satisfaction.
So previous hero could fight MC with the premise of her being disappointed and hurt by the fact that her companion chose the dark path in order to achieve world peace. Though naive, she has always believed in the inherent kindness of mankind, even if that was non-existent. Fighting MC would be hee trying to save MC from this torturous path of bearing the world's hatred for peace.
My other thought is that, she wouldn't fight MC. If my premis in the above paragraph doesn't hold, she might not even be another chosen of the Will. She doesn't have any reason to destroy her friend who is pursuing world peace in his own way. She doesn't seem like someone who would enforce her own idea of peace onto others. If she was revived by the Will, the Will won't be able to force her to fight the Demon Lord. She probably won't approve of MC's path, but at the very least she wouldn't interfere. She knows that MC isn't an evil person like what the other chosens think. The Will only chose those that has reason to fight and destroy the Demon Lord. If this premise holds true, the Will wouldn't revive her and even if it did, she wouldn't fight MC.