So my theory is that hes already been turned into the demon king. The title of demon king could imply to the strongest being currently alive and when he beat the former demon king he was awarded with the role/title of demon king
Shes been through a lot, and I think she maybe recognizes that the MC has probably been through some shit in his life and tries to comfort him-
oh and shes also a big tsundere
It's suprsingly very good, you won't be able to put it down once you get into it. It shows what pure motivation and hard work can actually accomplish it inspires me a lot to just keep doing my thing. Love it.