If Davy Jones was indeed the King of the AK, I'm saying it right here and now: Imu was born a low-class servant of the Davy Family, maybe even a slave. When the Twenty Kings turned against the Ancient Kingdom, he came to them as an advisor, as he had detailed knowledge of their inner workings.
Then at some point, he had whoever the user of the Ope-Ope Fruit at the time (Maybe a fellow ex-slave?) swap his and King Nerona Imu's bodies and then he killed him and took his identity.
All of the things Imu does is just to never have anyone know he was just a simple slave and thief once.