"From now on I'll call you Patience, my Little Brother. I don't care if you're human, hell I like you even more because of that. Once we get home we'll have a lot of fun, we'll play games until we fall asleep, and we'll probably get into a lot of trouble so you can rely one me. Call me Big Sister." 

The girl proclaiming Patience to be her  brother is named Charity, she was young only being thirteen, just two years older than him at that time, but she was already chosen to become the next Demon King.

The process in order to become the Demon King was fairly simple. A contest based on strength and strategy will be  held by the previous king in order to determine the next one. Charity managed to win this contest easily.

Contrary to what the title implied, growing up Charity was extremely nice to Patience never even once doing him any injustice, despite the difference in their races. She treated him nicely, even to the point of being overprotective.

The Demon nobles surprisingly kept quiet about their dislike of his existence, though that might be because they feared what Charity would do to them, if she found out they laid their hands on him.

Stolen novel; please report.

Growing up his daily routine mostly involved him honing his abilities with her. Of course she always held back when practicing with him, because compared to her, Patience was weak, fragile you could say. The sheer difference in their abilities was as clear as the night sky, but he never gave up. He did this to prove his strength, his worthiness to others, and himself.

Every day he kept training and training, even if it meant constantly breaking his own body over and over again. From time to time Charity would take him fishing, one of the very few activities that he mostly enjoyed, and after that they would go back to training. With every passing year his training would grow more and more rigorous but he managed to endure it.

At the age of Fifteen he was already considered extremely strong, even by Demon standards, though still not strong enough to make Charity, who was just newly crowned The Demon King release a bit of her true strength.

When he reached the age of sixteen, his efforts were noticed by the Demon Lords. Despite being only human, Patience was given a position, and an honorary title, the two of the greatest achievements a demon, much more a human could ever achieve.

He was given the title of Heretic, along with a position as one of The Seven Demon Lords. As one of the seven personal aides, serving directly under the Demon King, the most trusted and most powerful beings in the entire kingdom. With this there was no longer anyone who doubted his abilities, even the other Lords deemed him as their equal.

Patience enjoyed his peace as one of the Seven Demon Lords, until the war broke out.