You get the primary characters by unlocking chapters, there are 8 out atm and you get 3 on average for each one. After that about every other week there is a character event and you can unlock that character for a limited time. Eventually they release Lost Chapters which are from older character events and you can get them from there (currently 6). You get crystals pretty regularly and get tens of thousands from just playing the normal game mode, so dont be too concerned about what you have spent thus far. This is a pretty good banner to pull from so if you rush through a few chapters you can probably get a few of the 10 pulls (5k Crystals). Never do the single pulls for crystals, only use tickets for those (also given regularly for scoring in the limited character events).

You can keep an eye here for info on future events and banners as well as guides for character building. Good luck!