Fi

The origins of Fi and how she ended up possessing Flick's soul are an unfortunate mystery. While we don't have the full story, we can assume a few things from the circumstances. The pair have lost the knowledge of them being brought together due to the process having given them both amnesia. This is not even remotely supposed to happen, most demonic possession events can occur without issue, but something must have gone wrong. Whatever it was, it resulted in Fi going dormant for years, losing her memory, and wiping Flick's memory as well. Flick would continue about her life completely unaware of the demon.

When Flick was about 17 years old, she would officially "meet" Fi. Fi had been dormant for those 17 years, slowly regaining strength and magic to communicate with the human they inhabited. Flick always brushed off the voice as her own brain being stupid, but those thoughts were entirely discounted when the voice was suddenly as clear and present as if it were in the same room, not just thoughts in her head. As Fi continued to regain power, she simultaneously became incredibly attached to Flick. Fi, while having been much more vicious prior to meeting Flick, had become quite the motherly figure. Fi began funneling her power through Flick to protect the strange girl she was connected to, allowing Flick to wield demonic power through a demonically enhanced form. This would allow Flick to contain the same power Fi had, something Flick's otherwise regular human body was just not equipped to handle.

However, as time goes on and they've been in contact for 10 years, Flick's soul has become a little more demonic. This has been allowing her access to some demonic magic when in her human form, and giving her more ability to use that magic on her own. She's been able to use shadow magic without issue for some time now, something Fi has been more than happy to allow Flick to protect herself more often.