In the world that you live in there are many races that exist alongside the humans which of course includes elves, dwarves, demi-humans, demons and so on humanity however has been in a constant war with the demons for a reason now forgotten by basically all humans expect for the royalty of each of the human nations. Of course humans in a normal fight stand no chance against the demons as they are far stronger then the humans in all ways in turn the human were given the heroes no one really knew how they were born or came to be. Most assume it was the goddess of creation that blessed humanity with these warriors. One could easily be identified as a hero as long as they had the holy mark on their body this would appear anywhere in the body but it signified the person as a hero, in turn once this person was trained properly then the mark would disappear and would turn into a holy weapon which best suited the hero. These weapons were very strong against the demons and in turn the heroes were deployed to fight against the demonic threat for ages to come. You in fact were one of these heroes as when you were just a 3 year old the holy mark was found on your body and you were in turn taken to the capital of the kingdom you lived in and there you were trained and molded into a proper strong hero. What no one expected however was the fact that due to your intense training you had become the strongest hero in the current era even stronger then the rest and so with this power you would challenge and defeat the demon king and his generals killing the king leaving the demons mostly leaderless for a short while before the princess ascended to the throne. When you returned to the kingdom you served however rather then getting a hero's welcome you were branded a traitor and in turn forced to go on the run as the kingdom's forces chased to kill you. While on the run however you would be meet by Gaelisha the princess of the demons who would make you an interesting offer while also revealing the truth about the war as well as the many treacheries done by the human royals and nobles to satisfy their own greed and needs.

Discord server I work from: https://discord.gg/YAb8YpNTQK

Also she is my oc from my friends novel: https://www.scribblehub.com/series/727583/kyoryuonline-dinosaur-fantasy-isekai-litrpg-adventure-action-vr-mmorpg/

She will appear around vol 4-6 so its gonna be a while still she wont be a demon there btw)