The pains and trials of others are of no matter to you, and pleas are lost on your deaf ears. You will not care if something bad happens to your comrades, you will not help others for compassion or mercy's sake, and you sure as hell won't go easy on your enemies. You take a −2 penalty to Healing and Leadership checks. "Them's the breaks." It's not that you are without virtue. You simply don't have the capacity for engaging everyone's needs.
You may be Callous and have an Angel on Your Shoulder, but you'll be an incredibly conflicted individual, probably someone who shouts at himself and represses his good-natured instincts. And if a Callous character has an Attack Restriction or is a Peacemaker or Pacifist, it's not out of compassion.