Don't use flashbacks if you can help it. Flashbacks can certainly work, but more often they simply stop the story. Don't forget that your character has his own point of view and that how he reacts to stimulus does more to define his character than anything you could tell us by way of explanation. Enough of the kindergarten stuff. Let's move on.
People believe what they already know or what they want to know.
People rarely change their mind about a person just because the evidence says their opinion is wrong. If I really like someone the fact that you think he's a jerk bears no weight. It's the way we are. People are filled with prejudices of all kinds, and they extend beyond who we like and dislike to what we accept as true and accurate depiction's of the human condition. Recognition is a two- way street.