Fei is overall a good character and has some decent skills. Also, he's the only character with a dark combo and light combo. He's probably the best inside a gear, especially later in the game.

Elly pretty much relies on her attack spells, since she has low HP and defense. Has some decent combos, but nothing much compared to the others. Inside a gear, her Aerods attack can be quite useful to destroy many weak enemies at once.

Citan is probably the best character, being really fast (only losing to Emeralda), having high HP (loses to Rico later), healing and support spells that you'll have to use before getting Billy, and if you think he isn't strong enough bare-handed, he later gets to equip a sword! He's really good with gears too, but in this case he isn't THAT much better.

Bart is decent at attacking and has some nice skills, but his HP and defense aren't as good as Fei's. He's also pretty good inside a gear, especially when you get his Omnigear (his is the first one you get).

Rico has only a few skills and is incredibly slow, but has high attack and HP. His gear is particularly powerful, even if usually ignored by most players.

Billy has some decent combos with his guns, but his real prowess lies in healing skills. His guns are also very useful when piloting a gear.

Maria has a few Robo skills, but besides that she's really not useful. With a gear, it's the complete opposite. She doesn't even get an Omnigear but still kicks some serious ass.

Emeralda is quite fast, has a good number of attack skills, and some nice combos, making her a powerful ally in the field. Her gear is also very powerful and fast, even without an Omnigear upgrade.

Surprisingly, Chu-chu has some decent skills in the field. In her, umm, "gear" form, she pretty much sucks unless you give her an absurd amount of drives, which will make her quite powerful if you're a Chu-Chu fan. Personally, I refused to use her too much. I mean, come on, SERIOUSLY.