Refia is the adoptive daughter of a blacksmith in Kazus. She is known for constantly running away from her father because of how rigorous her training is. Thankfully, the one time she does run away in the game, she manages to barely dodge the curse that the Djinn lays upon the town. This leaves her free to find the key to defeating the Djinn and breaking the curse.
At first, Refia will leave your party after you have sufficiently advanced the plot. She will rejoin you soon enough, however, so don't worry about the lost experience. After she rejoins she will never leave you again.
You might notice that even as a Freelancer, Refia's strength is not up to scratch compared to Luneth and Ingus. Rather, she is better geared to being your White Mage and supporter in general. Her lower strength, defense, and hit points counts means that she struggles in melee. However, with enough investment and focus, Refia can be whatever you want her to be. If you focus her exclusively on various melee classes, she will become just as potent as your other characters.