You mean the angel in the animation said Charles was dirty? !
After his parents were killed, Charles was arrested by some upper-class nobles and held a ceremony of dark magic. At the ceremony of black magic, the nobles branded him on their chests as a representative of the lowly. It should be at the ceremony that Charles signed a contract with the devil in order to be saved.
This so-called "impurity" should refer to the young master's body polluted by black magic.