Demons exist and their power is beyond what humans can fathom. In fact they are so strong that humans do not stand a chance against them in a fight. The only thing strong enough to defeat a demon is another demon and it is trough this logic that Ryo Asuka hatches the plan to have his good-hearted friend Akira Fudo possessed by a demon. If a person is of pure of heart then he might be able to control the demon that posses him and thereby acquire power equal to a demon. (animenewsnetwork.com)