When you first start playing the mobile version of Tekken, you have access to just enough fighters (three) so that you can participate in every game mode. But do you want to stop there? No, of course you don't.
See, Tekken includes characters from throughout the console/arcade/wherever you play video games franchise, and the only way to really appreciate that fact is to gather as many of the dozens of fighters already in the game at launch. Tekken is a free-to-play affair, so you can definitely pay to unlock fighters faster, but we'd rather focus on the ways you can do it by not spending money.
First, though, let's answer a question you might already have.