Couriers are the characters that give the players rewards when they win a game. They can bring cards, Coins and Gems. There are different rarities of Couriers: Free, Silver, Diamond, Glorious, Mysterious, Epic and Mythical. Bear in mind that whenever you receive a Courier, it will occupy a slot. If your slots are full, you won't be able to receive new Couriers even if you win challenges.
Most of them are asleep so the first thing you have to do is wake them up, which takes some time depending on their rarities. Once you do that, you will receive the rewards they bring.
You’ll get the free Courier every 4 hours, just by playing. It has 2 special slots and is already awake. For this reason, it is not necessary to wait to receive the reward.