This game had over 1 million players in Japan. It came in English on Android devices around late February of this year.
The game is real fun, collecting cards, trading them. There are events which gives you chances of getting high rare cards. Talk to the right people and they will trade you in your benefit. Talk to the wrong person and you cant get ripped.
The game has a short tutorial, afterwards it asks for a referral code which gives you an Angelic Knight rare card and 100k to strart off. Mine would be STV35518.
The game gives you three chances to get rare cards without buying in-game coins. One is defeating the first boss and the other are getting to level twenty and forty.