Probably it is. The person maybe struggling with a stronghold. If the person is victorious, it is not necessarily a demon. Often, our walk with God is a struggle. We put on the amour because the walk is a fight( Ephesians 6:11) Satan constantly wants to pull us down,(Philippians 3:12). Behind every struggle there is a spirit. Illnesses could be a physical thing. Habits may also be physical-in extreme cases spirits are involved!