Aggressive Characters will threaten the player, but do not actually attack on sight unless provoked first.   Attacking the character or another, similar character nearby will provoke the aggressive character, as will simply getting or staying too close (if the Aggressive Character is making threatening "Fight me!" gestures, the Player is too close).  From that point on, all of the similar aggressive characters in the nearby area will act in a hostile manner (attacking on sight), until the player gets out of range.