Actually, the game demonstrates to you on how to acquire characters. However if you haven't gotten to that point or could not understand, characters are obtained by character shards.

Character shards are obtained by two methods:
1) Buying using real money
2) Getting secret letters

Secret letters are like blueprints that you use to farm for character shards, this is a grindy process and purely RNG based so expect to go through multiple runs. You can get secret letters from secret letter clues.