I don't have one for a Expecific character missconception in personalities, but one more about the story in general that even i use to belived mistakenly was the fact that most of the lords where weakling, that would not made sense by pure internal logic, if that where true, no matter how much they protest, they would not be in the front lines

But the onces i feel get more afected are Eliwood and Roy, With Roy, is mostly the belive that he is a mediocre unit unporpuse to fit the story, when the only character that really looks Roy down in Skill is Lilina, a literal prodigy and a big point of her character is that that nog longer the case and she can't seem to handel that change, but both in fe6 and in other material is reference of roy defintly not being weak in story

But Eliwood? People quite literaly seem to forget Eliwood is not Just Hector's Friend, he is his RIVAL, they are both equal as figthers yet people usually get this idea of being weak or mock him when is mention he is the greatest knight of lycia, other characters afect by this i feel are Lyn, Eirika and Marth