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Fang has access to the same Medic Abilities as Lightning, but because it's a secondary Role, she's even less useful as a Medic. By the time you are likely to learn these Abilities, the lack of a Curasa spell means Fang will be terrible at healing. Her only real advantage over any other Medic is that she gets Raise (which Sazh and Snow do not), but you're more likely to use Renew or a Phoenix Down late in the game anyway.
For a secondary Role, Fang is a damn effective Ravager. (This seems even stranger since she's the only Character who doesn't have Ravager as a primary Role.) She has access to every spell through tier 2, and while she only has two -ga spells, one of them is Aeroga! Perhaps her biggest disadvantage as a Ravager is that her strength is so high that she's more effective in other Roles. However, she does gain all four -strike Abilities, so even this isn't really a problem.
Though she lacks Vanille's Death Ability, Fang is perhaps the most effective Saboteur in the game, though her one glaring weakness, an inability to inflict Poison, is a problem against many of the toughest foes in the game. She also lacks the defense-lowering multi-target debuffs (Deprotega, Deshellga, Imperilga), but as a general rule you're only attacking one target at a time anyway, so this is not a big deal.
Fang is an interesting Synergist that can be quite effective despite having less Abilities than any other Character. She has access to Veil and Haste, but more importantly she gets the high-powered Abilities Bravera, Faithra, Protectra, and Shellra. These Abilities, exclusive to Fang and Vanille, are more powerful versions of the Abilities they are based on, but have a much shorter duration. You'll have to pay a lot more attention to your buffs if you use them, but the payoff can certainly be worth it.