I can only suggest two things about Billie/Anikh. 1) she has to play a part in moving the narrative forward, why did God put her there? How will her being there, causing mischief, determine the outcome of Joe's journey. 2)if she is an evil entity, her backstory should be consistent, was she a bad person? A killer. Another nagging issue, since we are delving into the supernatural, what are the laws of the universe that govern these players? As a scifi reader I am anticipating a grand ultra universe with forces at work trying to influence the outcome of our universe. This has good potential.
There will be some brief hints before she dies that all is not as it seems. Maybe she rolls her eyeballs up into her head, claiming it's a nervous tic. TBD. At first, Joseph will welcome her to his head. Who would do otherwise for a sweet little girl? Later, once her sinister nature becomes evident, there will be a discussion with God about why she's there. Apparently, it's to help Joseph found his religion, although God won't explain why. She does sh*t that Joseph wants but would rarely dare do himself. She also does things that will horrify Joseph.