If you like RPGs, final fantasy brave exivus (or something)
You can buy lapis to summon more, but you get half price summons once a day, daily lapis, lapis for doing events, lots of ways to get strong without spending a dime. You are not forced to spend a dime at all.
I haven't spent anything and have 3 top 10 characters currently (top 10 as in ratings, not top 10 in the server)