So, while Kingdom Hearts is full of them, probably the worst is Ansem.
The first Kingdom Hearts has you learning about a good wizard named Ansem who vanished in the wake of the current catastrophe. You are told that he may be key to saving everything. At the end of the game, we learn that he's gone evil, obsessed with doing anything to get to the biggest mystery of all, Kingdom Hearts, the heart of the universe, and chosen a dark path of evil to get there. He has been the secret ally of Maleficient, possesses the hero's rival and turns him SUPER EVIL, betrays Maleficient (!) and does big evil villain monologues about darkness darkness darkness. At the end, our plucky spiky-haired hero Sora calls his bluff that Kingdom Hearts is a black hole of infinite evil, and that indeed, the heart of all worlds is the purest light of all, and with that light defeats him.
This isn't necessarily deep, but it's consistent and acceptable for a final boss of a game aimed at kids.
So of course, Kingdom Hearts II reveals that no, that wasn't actually Ansem we faced in the last game. That was Ansem's apprentice, Xehanort, who banished his master to another dimension, stole his identity, convinced all the other apprentices to do an experiment with him about removing their hearts for SCIENCE, and in the process split into two beings, one of whom was the villain we faced last time. And that Xehanort was committed to stealing his mentor's identity for reasons that has never actually been clarified.
And the REAL Ansem then reveals himself to be the masked interloper DiZ who showed up at the end of the game set between 1 and 2, and has been pulling strings on other plots in the meantime, and then dies-ish because he's allowed his (rather understandable) anger at having been betrayed, impersonated and banished to another dimension, to go too far and dies of hubris.
There are bigger and weirder retcons, but this is the one that I think is the dumbest, because it really doesn't add anything of value.
The first Kingdom Hearts has you learning about a good wizard named Ansem who vanished in the wake of the current catastrophe. You are told that he may be key to saving everything. At the end of the game, we learn that he's gone evil, obsessed with doing anything to get to the biggest mystery of all, Kingdom Hearts, the heart of the universe, and chosen a dark path of evil to get there. He has been the secret ally of Maleficient, possesses the hero's rival and turns him SUPER EVIL, betrays Maleficient (!) and does big evil villain monologues about darkness darkness darkness. At the end, our plucky spiky-haired hero Sora calls his bluff that Kingdom Hearts is a black hole of infinite evil, and that indeed, the heart of all worlds is the purest light of all, and with that light defeats him.
This isn't necessarily deep, but it's consistent and acceptable for a final boss of a game aimed at kids.
So of course, Kingdom Hearts II reveals that no, that wasn't actually Ansem we faced in the last game. That was Ansem's apprentice, Xehanort, who banished his master to another dimension, stole his identity, convinced all the other apprentices to do an experiment with him about removing their hearts for SCIENCE, and in the process split into two beings, one of whom was the villain we faced last time. And that Xehanort was committed to stealing his mentor's identity for reasons that has never actually been clarified.
And the REAL Ansem then reveals himself to be the masked interloper DiZ who showed up at the end of the game set between 1 and 2, and has been pulling strings on other plots in the meantime, and then dies-ish because he's allowed his (rather understandable) anger at having been betrayed, impersonated and banished to another dimension, to go too far and dies of hubris.
There are bigger and weirder retcons, but this is the one that I think is the dumbest, because it really doesn't add anything of value.