You need ironite to play, at the beginning your natural sands of time refills are ok, but as soon as you start doing events they are not enough, so only use ironite to buy sands of time (sot) and play chasing those rewards that are possible for you. With some patience and a lot of playing events and completing all the worlds, you will be able to buy some characters with Frontier Coins if you want. But mostly your charactar are going to came from souls after playing a lot.