There are two types of currency in the game, these are coins and pixels.
Coins
You periodically receive free gifts which will usually give you between 100 and 200 coins (sometimes 500). Make sure you set your push notifications so that you get an alert whenever a free gift is available for you to collect.
Pixels
Pixels are earned each time you get a repeat character. A pixel gatcha machine will open once you have earned your first pixels and this will enable you to unlock more repeat Characters thereby giving you a higher chance of earning pixels.