There are three characters you can unlock in Dragon Ball FighterZ. To unlock them, you must follow the guide set up here. Alternatively, to unlock SSGSS Goku and SSGSS Vegeta, you can pay $2.99 and unlock them. Normal Android #21 is not playable, and is only playable in her Majin Form unless DLC says otherwise. We cannot say for sure yet.
Character
How to unlock
Android #21
Complete the Super Warrior Arc, the Enemy Warrior Arc, and the Android #21 Arc.