Be Nice is an area Special created by Kleya Smith. It inflicts damage to any target (even herself) that damages another. Secondary damage (such as shoving a person to the floor) is also considered damage and will cause Be Nice to hit the attacker. In addition, it boosts her HP (possibly at the expense of her Stamina, an in-comic speculation).
The damage is equivalent to the amount that the target caused. It will also damage the Happy stat with the same amount of damage if the victim is signaling that he or she is unarmed (raising of the hands).