Frederika (Halloween) is part of the limited-time Halloween units, alongside Little Leela (Halloween) and Rairyuu (Halloween). Her main job describes her as a unit, she's a Duelist that's good at 1v1, whether it's against tanks or mages.
She has MANY tricks up her sleeve, but whether the auto AI will use it effectively is yet to be known:
Good amount of evasion from passive and buff + blind from LB + accuracy down from her counter ability.
Can increase single target and area attack resistances, which are much rarer to get penetrated/reduced.
Main attacking ability comes with a drain effect for some self-sustain, and it can reduce enemy DEF for high damage.
10% base magic resist + 30% from job lvl 25 ability to have 40% magic resist to go against mages.
Against other non-mages, she has some Slash and DEF penetration as well as the -DEF ability to be decent against tanks. But mostly relies on evasion.
If all else fails, she has Dragoon sub job with some jump skills to avoid getting hit.
Main downside is that her buffs are separated into many skills, and her attack range is only around mid to low. However, she does have access to a +1 MOVE and JUMP from passive, as well as another +1 MOVE from Duelist sub job, so she won't exactly have issues closing in. But having 4 different TP skills for different situations might require tuning pretty often.