As a person of unknown age, but judging from size,it could mean Fei could be as young as 11 or 12, but no older than 17.Fei has shoulder long golden blond hair, and green eyes, constantly wears a green scarf.
Fei cares deeply for those who have gained her trust. Such as when Ruri got taken by Kenmi and she offered to help. It is shown Fei doesn't like to do nothing and pretty selfless as shown when she ran across the fight between Hon and Knight to bring the serum to him. In episode 12,after gaining Kenmi and Hon's core she wanted to destroy everything because she felt like everything that heppened is her fault,but with the help of Alma and Knight she was brought to normal.
Just like Knight and Hon,she was an experiment in Kenmi's laboratory. Although after Knight managed to escape he took Fei with him , but not Hon after witnessing his transformation into a Darkstone and thinking he was dead. After many years after their escape her and Knight have been living secretly seeking for their freedom and revenge.