Battle
In battle, Fee is a very strategic character. She relies on her special abilities to inflict the most damage, and when you're attempting to rack up combo bonuses, she's one of the first people you should turn to for her quick and multiple-hit attacks.
Okay, I admit. I never used Fee much in battle; my battle party, for the most part, consisted of Gray, Felt, and Noin. The only time I used Fee was in the final battle.
Since she needs meter to be truly effective in combat, it's generally a good idea to not have her and Poe in the same lineup, as they both consume meter. She is best put together with characters who can deal enough damage and build up the meter, effectively being "batteries" for her to deal out the hard stuff.
Fee's Charge command has a wide range, wider than a lot of the other characters. Also, her Break command connects twice, unlike the others.
Fee has one of the highest magic ratings in the game, and has the ability to revive allies. Though, you have to wonder why she has such a high magic rating if her most powerful skills are physical-based.