The most straightforward method is to purchase characters at the in-game shop, located at the Shop & Customize part of the Main Menu. But fear not, there are no microtransactions here, as everything’s purchased with the Zeni currency you earn through battles.
Aside from needing to gather lots of Zeni to acquire the characters, there is one more important catch—not all the characters are listed here at first. The shop updates with new items as you increase your player level, and that includes new characters. Every unlockable fighter should be in the shop once you’ve reached Player Level 20.
One of the quickest ways to increase your player level is to wish for it when using a Shenron summon. This boosted me up to level 8 and updated the Shop quite a bit. You can also wish this with a Super Shenron Summon for an even bigger boost, but there’s arguably a better use for that dragon that I’ll be going over soon.
I also want to reiterate that although you can just buy them all in the shop, there’s always an alternative. Aside from the eventual DLC characters and Goku (Mini), the latter of whom is exclusive to the pre-order pack, you can unlock everyone through other means.