To me, a bad character is where they are more trouble than they are worth on the field. Boah from Fire Emblem (and Shadow Dragon) is a good example because of his abysmal growths. Matthis wasn't much better, because of his growths being consistently terrible in every game he's been in, and his AI tending to kill Lena.
Dorcas was already mentioned because of his poor growths, and a character with poor or inappropriate growths, i.e. a magic user with strength can easily be a deal-breaker for me.