Your friend code is a special code equipped with all Wi-Fi games. If you imput a new friend code, that person becomes your friend. In MK, you have to race them to get their stats and pic. In MP Hunters, you can have enemies too if you battle them and after, ask them to be your enemy (they can say no). When you have a friend or enemy, you can lock them so you can't accidentally erase them. I hope that helps.