Every so often, you come across a game that you just think... wow. Sid Story can entirely be free to play if you so wish. The event enemies, Exceptionals, change every two weeks and have beautiful artwork. Most high-level players accept new player requests, so defeating them is no problem, as they get sent automatically to friends. If you are able to join a good Circle, which are the alliances/guilds in the game, you can get a ton of free Gems per week. Along with that, every 15-30minutes, you can watch an ad to obtain 10-15 crystals, along with completing "explorations" (which are things that require waiting a certain amount of time to finish) you can get Magic Papers for the Forest, Gems, Jokers to fusion max 4*s, among other prizes.
Its a very fun game for even free to play, and Id recommend it to anyone !