Lloyd: As per Anime Law for any hero who is not seriously psychologically damaged, the hero of the game is a good-natured, moral, yet utterly moronic simpleton. I'm really starting to wonder about Japan, at this point. Does it just never occur to them to try starring a character who is both sane and relatively smart?
Colette: In a small twist of character development that is about as close to originality as this game gets, the main hero's love interest is actually just about as nice-but-dumb as he is. Whether it's natural as with Lloyd, or simply brain damage as a result of all the clumsy accidents she has, is hard to determine. Either way, though, clearly a match made in heaven.
Genis: "Gasp! My new friend has betrayed my trust! I feel such a conflict of loyalty now--should I side with my honest and caring friends and family, half of whom I've known all my life, or with the manipulative traitor that I met about an hour ago? I CAN'T POSSIBLY DECIDE!"
Presea: Once her 10 minutes of plot are over, they're over. Frankly, you'd think that the game could get a lot of mileage out of an adult woman trapped in a super-strong child's body whose growth was stinted by a magical doohickey that also robbed her of her free will and personality for several years, leaving her alone and a temporally-misplaced freak of nature, but...they don't really seem to bother. After a little bit of remorse over her dad having died while she was zombie-ing it up, her only real role to play is just taking part in another character's development.
Regal: A backstory that's way lamer than it should be makes the only notable part of this character his fighting style--basically, he vowed to never use his hands to fight anybody, so he only uses various kicks and such in battle. That sounds kind of neat and all, but honestly, I gotta say, if you're in dangerous combat where the lives of you and your friends, as well as the freedom of millions and that of future generations, are on the line, it seems a little stupid to purposefully hold back.
Sheena: Much like Chisato in Star Ocean 2, Sheena has a strange kind of appeal as a character just by being a fairly normal and non-annoying character that makes her my favorite here. It's not that she's a good character per say--it's more just that she's really the only one that isn't a bad character.
Raine: For a character with a crapload of exposition, Raine doesn't have a whole lot of personality. When not fulfilling the role of "Character Who Is Knowledgeable About And Likes To Explain Plot Stuff," Raine's only real distinguishing trait is a mania for archeology. It's not even portrayed well enough to be as humorous as it should be (and that's not much); she never gets into enough detail or mumbles enough history about the ruins she's studying that you actually buy it. Will from Tales of Legendia's obsession with animal physiology is a much better execution of the same joke.
Kratos: Congratulations, Tales of Symphonia, on creating the most average Swordsman character of all time.
Zelos: You know your game's in trouble when the deepest character in it is a shallow playboy.*
Mithos: Y'know, I love my sisters a lot. But I have to say that if one of them died, I wouldn't split the world in half and create a military religious order that controls and enslaves countless people of other races, ruining their lives and their descendants' lives for centuries, with the intention of finding a human sacrifice through which to revive my sister. Maybe I'm just a bad brother, though.
* Final Fantasy 9 notwithstanding. Although Zidane isn't a shallow playboy to begin with, so it doesn't qualify anyhow.