HI NO KAGUTSUCHI

Hi no Kagutsuchi summary:

On a trip to look for his brother, a hungry Hachiya Rei stops at a shrine to pray. It is just his luck that he is accused of stealing money from the shrine and forced to work to pay off the amount stolen. However, the real thief turns out to be a daemon, or a god turned evil! An encounter with the daemon unleashes Kagutsuchi, the god of fire, concealed in Rei. As Kagutsuchi's proxy, Rei can use the god's holy fire to purify daemons, but there's a side effect...