oerba yun fang
At first, Fang's story and motivations are unclear, making her seem to be an enigmatic character. As her influence in the plot increases, we find that she's a strong warrior and an alluring woman. She's confident in herself and in her abilities and sure of her goal: to do whatever it takes to save Vanille from becoming a Cie'th. Though she is introduced as a l'Cie helping the Sanctum, the truth is that after the events at Euride Gorge, she was picked up by the Cavalry, who plan to oppose the Sanctum.
It's when Fang meets Lightning that she explains where she and Vanille come from: Gran Pulse, the untamed world below. They were crystallized during the last war, five hundred years prior, and only recently awoke in Cocoon. Fang remembered nothing of their Focus, and in her frantic quest to determine what Vanille's Focus is, they attacked the fal'Cie at Euride Gorge. As a result, Sazh's son Dajh became a l'Cie. They were also responsible for Serah's becoming a l'Cie, presumably in their stead. Fang feels better about it after she confesses to Lightning; unlike Vanille, she doesn't feel the same guilt for their actions. She recognizes that they made a mistake, but she doesn't dwell on it, preferring instead to face forward.
Unlike the other members of the party, Fang's l'Cie brand is white and doesn't change, indicating that her Focus is already complete and that she has no fear of becoming a Cie'th. Ironically, though, Fang is the one who worries most about the others' brands, frequently checking Vanille's to see how far it's gone. In the Fifth Ark, Fang readies herself to fight against the rest of the party, determined to complete their focus even if it means destroying Cocoon. She is unwilling to see the others turn Cie'th, no matter what the price. The appearance of her Eidolon, Bahamut, forces Fang to face the reality of their situation and to accept that fighting alongside the others is the only option available.
On Gran Pulse, Fang learns the truth of their Focus from Vanille and helps her defeat her Eidolon. She gives Vanille the comfort she had needed ever since she lied, telling Vanille she's sorry she did it for Fang's sake.
In the end, Orphan tries to get both Fang and Vanille to become Ragnarok, but to no avail, as Vanille and the others escape from the illusion of becoming Cie'th and restore Fang's will. After Orphan's destruction, Fang and Vanille become Ragnarok, their combined will enough to save Cocoon. They become crystallized together, just as they were in the beginning.