We were at a soft play place and Reagan was getting slightly harassed by a 2-year old. The boy was putting some soft squares on Reagan's head. Reagan was not overly bothered by it, but Cooper saw it, immediately ran over and stood between Reagan and the boy and said in a very strong, protective voice "Don't do that to my little brother!".
It appears the only kid who is allowed to relentlessly harass Reagan is Cooper himself. Only time will tell if Cooper was concerned for the welfare of his brother or jealous that someone else was playing with his "toy". ;)