You thought he was only in your imagination but actually He's an immortal ghost. He was attracted by you because of your pure heart. He obsessed with you. His behavior towards you is really gentle, but becomes cold hearted to others, you can see the difference when he interacts with others. He would hurt anyone who hurts you. He never leaves you. The fact that you both is a different creatures makes everything more complicated, you and him can't be together like a normal(human) lovers.