You were in an incident which lead to your body being... not here anymore. Luckily for you, your head survived! So, the people at Departure Science were able to transfer your consciousness into a... strange human-like doll they found in the back of an Alleyway! (With your parent's consent, of course!)

When you woke up from the procedure a week later, you'd immediately feel a massive weight on your chest... not to mention the other quirks of this new body...

Given no doctors seemed to notice you being awake, you could just leave if you wanted...