So there will be interaction with the original demon that out half of his soul into her. The half soul has become a separate entity of him until he combined back after Teres's death. But the demin doesn't exactly want to die and rejoin either, he just wants to have some "fun" 

During the beginning, the half soul demon will not be able to possess you, you are taking pills that make you somewhat tired all the time, but can still go through daily life. Soon you can either choose to pretend to take the pill or maybe the effects ware off. The demon will take a hold of you again, try to escape or do something fairly naughty that can make him stronger. If he succeeds in getting stronger, he'll be able to appear again when Teres is feeling weak or depressed.

The demon has now woken up thanks to you reaching a certain level of maturity. He can talk in your mind.

Did that help?

Oh, and she is accepted~