Worst character, hmmm... tough call. I like a lot of them (including most of the main cast of V, which I found very well-written), and can even find something nice to say about some of the more worthless ones - like Esmeralda, who's at least part of a decent magic unit. (And I can't hate Qlon - he seems friendly, and when Max and Sancho join, he's in the best Charge unit, bar none.)
(I can't remember if Taggart is in a particularly well-powered unit, himself... I hope so, because I'm pretty sure he doesn't fight or do anything else of note aside from tell you Warren needs rescuing. And then Warren contributes nothing.)
Of the Suiko I crew, then... I'll go with Ledon, Krin, and Giovanni. Sure, they start out being... SLIGHTLY useful in the Thieves' unit (when they don't fail) and helped me out a little in early battles, but they fall by the wayside. None have particularly endearing personalities, and Krin kinda sucks in battle. His saving grace is his permanent Escape talisman, but Stallion has one too AND lets you dash AND isn't as terrible in battle. Well, I guess Krin at least lets you into Lepant's manse, even if he tries to sell you out later.
Also, Sanchez, for very obvious reasons. "I'm old and set in my ways. Sorry." Aren't there characters throughout the series, as old as and older than him, whose minds change? Even in the same game? What a load.
EDIT: Wait, yes, of course. Liukan's RIGHT THERE. HE'S older than Sanchez, and was able to be convinced to come out of retirement and serve the Liberation Army, although I guess being kidnapped by Milich and being there when Gremio died MAY have helped. Either way, Sanchez really has no excuses now.
Suiko II... Jess. That business he pulled with the enemy camp and not even being concerned for Jowy's safety pissed me off something major, as did his recklessness. Not to mention the whole Annabelle ordeal. Which, hey, she was awesome. I'd be pissed at her murderer, too, but he just throws around accusations blindly and goes "LA LA LA NOT LISTENING" to anyone who may say a word against them.
Marlowe's just... pointless. What the hell does he even do aside from serve as a Star of Destiny? And he's got a thing for a 7-year-old girl. Wow.
I'd also add Zamza, because whoa, what a huge prick! But I recall him actually being a fairly good fighter, it's just I can never stand the thought of using the little snotrag.
V, well, I can't really call Salum the "worst", because he made for a nasty, compelling little thing. He was well-written, and kind of fascinating to watch, JESS. As a person, God, he's horrendous. But as a character, I love to hate him. Same for Euram, who at least has the chance to undergo character development later on and become a better person. I think the worst would be Urda. I mean, she's an alright fighter, it's just "barbarous human this, barbarous human that, Isato Isato Isato Alseid Alseid". Too bad she didn't live in the Village of the Elves instead of Alseid. I like to think she was visiting when the Burning Mirror struck, though.
(I haven't played III recently enough to remember its cast well, and IV's was just pretty much completely unmemorable. I mean, I liked Elenor, Lino, and Aldo, Jeane and Viki of course, but damned if I can remember anyone else.)
ETA: Apple has her uses, too, but I just realized that she essentially sucks throughout her appearances as well. She never really comes into her own. In I, she just hates you and clings to Mathiu. Okay, fine. Hardly the most one-dimensional, obnoxious person in that game.
II, she helps out some, but tends to really fark up anything she takes charge in. She at least admits she sucks, but puts herself down so much it feels like she's fishing for compliments anyway and seems content to rely on others without bothering to observe and improve. Sure, brains like Shu's aren't necessarily totally acquired, but...
I don't recall her role much in III and if she actually does do anything noteworthy. I recall it all being pretty much left up to Caesar, though, with Apple tagging along to keep him in line.