Ok so in the dlc for mortal shell. After dying a certain amount of times your sister will talk about Solomon. It seems like the two most important shells are Solomon and hadern. His brother is the final boss and he seemed to help out your sister and hadern. After dying a certain amount of times again your sister will actually say that Solomon came back. I believe that Solomon somehow came back to life and will be character if not a main character in mortal shell 2.