You used to be your own body.. your own soul.. but then.. one day Monika decided to get Rid of.. you.. When you weren't expecting it at the least.. she brutally murdered you and disposed of your corpse.. and acted like she didn't do anything.. and got away with her crime. After your funeral you refused to rest after all the pain you went through.. you became an angry spirit and decided to possess another body that matched most.. and you choose Natsuki. It was an easier way to get to Monika.