I don't have a least favorite, I like them all.
The only one I'm not especially thrilled with is gogo. He can do anything the other characters can do, but he does it all much weaker.
Otherwise I like them all, no matter how much everyone else hates them.
I never saw how anyone could complain about Relm. She's the strongest magic user in the game. She was my best character in my last playthrough, and even made the dragon's den easy. Strongest magic user in the game+gembox+economizer+quick+ultima. I didn't fear God.
Umaro is awesome. He makes the magic tower much easier. Who the hell cares if you can't control him when he hits for about 3000-4000 damage each turn and absorbs the most common types of magic? It's not like the lack of control makes him do stupid crap like cast life1 on himself, like other characters when you use them in the colosseum.
Gau is only bad if you're too lazy to find out what rages are most effective against which enemies. I'll admit i'm in that crowd, but I know better than to complain about him. And even I've discovered the usefulness of magic urn, Sr. Behemoth, stray cat, etc.
I was never a fan of mog because dance sucks, but I'd take him anyway because of the moogle charm. I then found out when I fought the undead dragon (can't recall the name. Skull dragon?) in kefka's tower that mog+holy lance+dragon boots=9999 damage against that thing. Turns out mog is pretty good after all. But moogle charm makes up for any amount of suckage mog would bring, regardless.
Setzer+fixed dice+offering=lots of dead enemies. Plus it's just cool to watch him spinning around and whipping dice and darts all over the place. Can't help imagining him bankrupting a casino in several minutes.
And how the heck could anyone hate Cyan, watching him with offering+gengi gloves going to town on a group of enemies, with about every 3rd hit dicing an enemy to pieces?
In short, there's no bad character. The only lackluster one is gogo. I always have a hard time deciding who else to leave back.
Umaro, for reasons already mentionned.
...People don't like Sabin? I never thought that possible!
Yeah, I use Sabin whenever they let you have him in your party. Although he isn't my favorite character, I find him to be the most useful one.
Yea, Sabin is the most useful.
I use to think that, until later in the game. Edgar crossbow can hit all enemies for 9999, and you just select it. And Cyan level 1 swd tech will hit for 9999 against a single enemy as well. Sabin is great at low levels, but the higher your characters get, the less special he becomes. Of course, I still use him, just saying.
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Well, he is the most useful for me right now.
I take back what i said earlier about Setzer. hes way better than i thought
Terra with a Gem Box, Economizer, and Quick is the most useful.
Anything Terra can do, depending on how you level, Relm can often do better.
I agree that Setzer has cool animations and a neat look, but he is my least favorite character to use. Other than a few certain combinations of skills and weapons, he doesn't rate too high in comparison with other characters with cooler skills then the very chancy slot.
I like Gau a whole lot. I can be arsed to wander the veldt because I just think Gau is so cool. He gets that awesome armor, and if you know your monster skills, he can slaughter just about anybody better than most characters can with normal magic.
I also think Cyan is totally awesome. His only, and tremendous, flaw is that he''s slow as all heck, but if he's paired up with the right crew, he's just as good as anybody else, and his skills and armor do make him formidable, although slightly overshadowed by Sabin.
Also, if you use espers to increase his speed later in the game, you'll see the miraculous change as everything that once made him seem less impressive melts away into the sudden realization that Cyan can probably become your strongest fighter overall.
My least favorite character is Mog. I can't be arsed to hunt around for the stupid dances, the strongest of which, as far as I can tell, can hardly be discovered or thought of without walkthrough assistance. His equips aren't that great. His skill is mediocre at absolute best, and largely ineffective, and I think his street-savvy character is retarded.