Duel Coins, or the pink cat merchant, are used to purchase cards or skins for your characters. Duel Coins do not expire. Gems, coins, and characters are earned through achievements and cannot be bought with currency or Duel Coins. Check out our community Steam forums for sales and special edition card announcements.
You can purchase Duel Coins by clicking on the pink "+" icon in the upper left corner of the screen. Prices may vary by region.