Filia is a minor character in Witch Spring 3. If you never enter the Red Fox Forest where she lives, or don't explore the area much, it is possible to never encounter her. However, she will appear in the after story regardless of whether her story was completed during the main story.
The first time Eirudy encounters her, she appears to be a strange-looking fox in the Red Fox Forest. When Eirudy approaches her hiding spot, she runs away.
The second time Eirudy enters the Red Fox Forest, and goes to the back left-hand corner of the map, she will notice a teleport circle much like the one at Wardilum. There, she sees the "strange fox" again and identifies her as a deity. Filia, surprised that she was found out, stands up to reveal herself.
If Filia's story was completed during the main story, Eirudy will encounter her again in the same place where they first met. Bemoaning that no Warriors were appearing anymore to take things from, Filia only now learns about the end of the witch hunts. Dismayed that she's lost her livelihood that she's known for most of her entire life, she hesitantly takes up Eirudy's offer to live with her.
Depending on what has been achieved in post-game before this, Filia's new living space will change. If The House Gifted From Adrian has not been unlocked, Filia will move into the Doll Storage in the Marionette House, to her slight displeasure as she finds the dolls creepy. If the house has been unlocked, she will move there instead.
Filia uses vines in battle, which are capable of restraining her opponents.