Lightning. A complete joke of a "heroine". No charisma, no personality, no motivations, unrelatable, undergoes no development, an utterly blank slate who is somehow excused by much of the fanbase simply for being a "badass with boobs". It doesn't help that we now have 3 games devoted to such a worthless character.
Vaan. Tacked on to perpetuate the shallow whiny emo boy JRPG protagonist stereotype, because that's what Japan wanted more of apparently. Nothing else to really say about him.
Honorable mentions: Everyone from Kingdom Hearts and the Tales series.
After being used as a pawn by his master to literally wipe out an entire town and all of it's inhabitants, and then right after being confronted by a person that explains that you are a carbon copy of him and stole his life, and that your life is basically meaningless. On top of the entire cast throwing him to the curb for a bit, him changing his ways was well executed.
The problem was it wasn't so much "changing his ways" as "completely wiping out his personality". There was no development, no inner struggle, there was a group of horrible characters treating him like a monster (which is rich when all of them were monsters themselves...looking at you Jade and Anise) and tormenting him into becoming an empty shell. He was a completely different character after he cut his hair, which is terrible no matter how you look at it. "Changing" shouldn't t involve killing yourself from the inside.