My Story Up to Day 13 in This Game

I'm having a tough time progressing because I didn't look at any guides at first and used up the coupons that give legendary characters. ㅠㅠ Most of the coupons I tried to use after searching online didn't work. Their expiration dates seem to be less than a month. The game itself has its unique fun, though. It's just that there are too many package deals. I've gotten 2 Sacred Shards so far (from the campaign). I'm currently trying for one from the progress missions. But even after using a Sacred Shard, I haven't gotten a good character yet.