NK Coins (Ninja Kiwi Coins) are the currency used by Ninja Kiwi that you purchase with real-world money or earn by doing tasks. They are used in Ninja Kiwi games to purchase premium upgrades, or simply more in-game currency. They are used in Bloons TD Battles to buy Medallions and to increase Battle Energy, used in Bloons TD 4 and Bloons TD 5 to purchase upgrades such as Healthy Bananas, or in Bloons Monkey City to purchase premium buildings.