If they were to add another fairy I'd say the most appropriate would be a fairy that lowers enemy resistances, perhaps a spell that resets resistance to disease and removes immunity to virus. A spell that lowers elemental resistances. A spell that reduces resistances to other jobs debuffs. Maybe a high end move that removes an enemies immunity (but keeps it highly resistant still), it may not be a stun but it could allow for a stun on stun immune mobs, even if only for a second. And a spell that raises party members resistances to certain effects. Although that might be better on Selene.
