The Spy card can be used to join a battle or initiate a battle. It is not used as a champion. To initiate a battle, on your turn play a spy card and state any number card you'd like to battle for. This allows you try to steal cards you may have seen another player win in a previous round. To join a battle you aren't already in, play a spy card before players reveal their champion cards and now you can compete as if you had cards at stake in the battle.