For the new guys that don't know, Haru has a dark persona that is Kill Joe.

Kill Joe was a infamous serial killer before was executed, and years later he became a ghost that posses Haru and take over his body any time someone kill the young dragon.

When Haru dies, Joe take over his body, and when someone suscessfully kill Joe, Haru comes back to normal without any memories of what Joe did. Basically as long Joe is still in the blue dragon's body, Haru can't die.

I like to think that they are my version of Eddie Brock and Venom