The "main" mind is what you can call the core of the Elliott. He is still inexperienced and was extremely lonely, so for a long time, to retain his mind he entertained himself with his childish actions.
For a character that is around 17-18 5 years means more than one-third of the time they had a sense of self, so I can understand if you find it still silly, but I think that it at least makes sense.
Once more I really appreciate your criticism, thanks for reading and have a great day!