Edward, the originalSpoony Bard, had horrible skills, no decent parameters and was overall a very whiny character. But he was improved so much in each remake! The GBA remake increased the chances of status effects, making his skills actually be a legitimate threat; the DS remake included a pleasing design, genuinely useful abilities and a goodvoice actor.
Edward is the weakest character at low levels (which in the original game, was the only time you could play as him). Once properly leveled (which is really only exploitable in IV Advance), he's the strongest AND the fastest.
Final Fantasy IV's Golbez was not loathed, but was seen as a generic Darth Vader ripoff and viewed as thoroughly less interesting than Kain and even the Four Fiends he commanded. The reveal that he was just being mind controlled by the Giant Space Flea from Nowhere had the effect of turning him into a non-entity with no agency, and The Reveal that he's Cecil's brother is followed up by very little and is wasted (in part due to the technical limitations of the time making it impossible to include as much dialogue as they wanted). The DS remake re-added many of those scenes (in particular, a crucial early scene concerning his backstory is included). In Dissidia, he's allowed to have a relationship with Cecil based on both of them knowing that they are brothers and having to reconcile being on opposing sides, allowing Golbez to come off as one of the more nuanced and developed characters in the whole game, and finally paying off on some good concepts that never got explored in the original.
Kefka was considered a boring, underwhelming villain in the Japanese version of the game, where he comes across like a fool with no idea what he's doing, and a Conflict Killer who cuts short the more interesting struggle against Gestahl. But in the West, his dialogue was rewritten extensively to make him into a monstrous, nasty sadist with a genuinely charming sense of humour, who's still regarded as one of the best villains ever to appear in the medium, let alone the series. This was so much an improvement that his American characterisation was ported back across the Pacific for Dissidia: Final Fantasy.
Cait Sith was a damn near reviled creature by pretty much any player in the game (and very few people play him if presented with an alternative). But he was much more positively received as the one bright spot in the otherwise very grim Dirge of Cerberus.
Genesis made his first appearance in a laughably terrible live-action cutscene in Dirge of Cerberus, was a shallow Sephiroth Expy, and was promoted as an Ink-Suit Actor for Gackt. Accusations of him being a Canon Sue abounded and were not without merit. While his next appearance in Crisis Coreremained contentious due to his constant recitation of awful poetry and his inclusion in that game's version of the Nibelheim story, he's got fans even amongst people who hate most of the Compilation due to the writers using him to allegorically discuss the good and bad of Final Fantasy VII's Fan Dumb, with his plot being based around overidentifying with a bad fantasy story and trying to find the truth behind its mysterious open ending. Some have also noted an element of Stealth Parody or even Writer Revolt about his portrayal, which is so over-the-top and full of uncool Bathos that it seems that the writers were mocking his forced inclusion.
Squall was often referred to as nothing but emo and whiny (and had the misfortune of headlining the game that followed the massively-popular VII), but thanks to his role as a leader who actually helps the protagonists and good voice acting in Kingdom Hearts, Squall has become a deeper character. Compared to the below-mentioned Dissidia examples, it's hard to say whether the series improved Squall there, too, since the first game went with his originally anti-social personality and the voice sometimes didn't match his lines. (The second, however, had Squall play off ofhis old man Laguna in one of the new cutscenes, where his voiceactors made an effort to sound younger and better match Squall's age in FFVII.)
In general, most of the characters were pretty unliked in the beginning of the game, but improved as the story proceeded.
Hope was hated by Fans because of his constant wangst over the death of his mother. However, he began to mature over the course of the story, stopped panicking over everything and buried the hatchet with Snow, after confronting him in Chapter 7.
Vanille's Genki Girl personality and Australian accent made her difficult to like at first, but as it was revealed more and more that she's a Stepford Smiler and her major breakdown in Chapter 8, most people warmed up to her. Her awesome scene of standing up to the Final Boss helped, too.
Bartz, while not a bad character overall, was seen as dull, even by the standards of preceding Final Fantasy protagonists. His adversary being, well, a tree... didn't help. In Dissidia, Bartz was given a balanced appearance between his forgettable sprite and Amano design, he became a much more likeable character, and his fighting-style involves dual wielding two of the most badass weapons in the series. And Jason Spisak does an amazing job at voicing him, along with imitating other character's voices, when using their weapons.
Like Cloud above, Sephiroth started out as ridiculously popular, but Villain Decay, terribleMotive Decay and cynical fanservice hit him hard, and fans begun groaning every time he showed up to spout the same six lines about despair so Cloud could hand his ass back to him. While the vanilla Dissidia fits perfectly with these trends, his quest plotline in the prequel Dissidia 012 gives Sephiroth a Character Rerailment. A lot of fans regard his suicide in this game as one of his best character moments in any of his appearances, since his reasons for doing it have a lot to do with his own personal character and nothing to do with being a plot point accessory for Cloud.
Jecht was rather one-note as Tidus' opponent. In Dissidia, however, his relationship with Tidus was made much deeper and improved voice acting has also improved Jecht's character to most people.
Lightning was, and continues to be, disliked by some people, owing to the very divisive nature of theLightningSaga. In Dissidia 012, her personality is based more on how she was in the second half of XIII: she doesn't tend to punch people and, while still depicted more as a loner, is much more open to depending on others when necessary and ready to trust in teamwork. Unfortunately, her role in the 12th Cycle and treatment as the main hero of the story, thus supplanting the Warrior of Light, only furthered longstanding accusations about Lightning's status as a Creator's Pet.