One of my characters Tom-Tom and Corde.
The idea behind them is that this little girl is a victim of war, specifically at the hands of soldiers who abuse her. In her last moments she makes a deal with her imaginary friend Tom-Tom that she would give anything to live and get her revenge. Well imaginary friend wound up being a demon that shares her body when she agrees, healing her and giving her the power to tear apart anyone she wishes, which she embrasses with childlike glee.
Character wise Tom-Tom is less a blood raging demon and more of a creature that likes to experience the emotions of humans and Corde gives him so much love and respect he comes to love her in turn. He doesn't want to take over her body entirely, instead he serves as a guardian that protects this poor child from friend and foe.