I think people view character as a player being good off the field only.
Nix is looking for guys with football character, meaning that the player will breath, eat and sleep football 24/7 and not be distracted by his latest contract, the car he just bought, the crib he lives in or the fame of just being an NFL player. When you draft players you think you get that guy, but for the most part the player doesn't become "a good one" because he doesn't care much for the hard work he needs to put forward to become better.
That is why I think interviews and background checking is very important when studying a prospect, they need to have the production but they also have to show the drive to be better by studying film and working in the weight room.
It doesn't matter if it is Newton, Fairley or Peterson, you have to try and find out that their goals include being the best they can be at football.