Dead Soul Revolver(2008)

Dead Soul Revolver

When a person who under goes extreme depression/anxiety loses control, they will become a demon consumed by darkness. Luckily there are those devoted to hunting these demons. Kagami Kenichi was taking a shortcut home from school until he gets attacked by a demon. A girl named Miyu appears and fights off the demon with her gun. Kagami's normal life comes to an end as he ends up making a contract with Miyu by kissing her, causing his soul to become her source of power and her gun to get bigger and stronger. Kagami soon comes to realize that humans actually turning into demons is real!