I just recently started on character design which has always been a bit of a weak point of mine. Having a character be ridiculously incompetent is generally bad but making a Mary Sue is worse. Some resources i have been looking into to make my NPCs seem realistic and memorable.
Fantasy Character Development Help
Character Development Tips
I'm still looking for advice on the creation of realistic characters and am spending time rereading novels or watching movies to get a good idea on how others have designed their characters.