Threre is a novel where MC ( not BTTH) have flames and he is able to steal them from others. I remeber that the color of flame is important in this world. He is hiding his ability becouse the ( flame clan ? , divine ray clan ? dn) are gathering flames from others. ( what i remener is that he hit to girl and her master ).
It is not that. What still remember that the MC had a master who was hiding in the cave, he was fortune-telling, and belong to undead race (unless I am thinking of 2 different novels) you're on the alert as always, but that's not it