In addition to the roster of playable Champions you start with, some Champions can be unlocked by completing specific objectives.
Conditions for these objectives can be found by tapping the icon to access the objectives menu at the bottom of the home screen.
Unlockable Champions can be found in the CHAMPIONS tab of the JOURNEY menu.
Here, you may view the various unlock requirements. Some Champions require a one-time action or achievement to take place, whereas others may require multiple.
For example, you may need to open a Free Crystal or win 20 Arena Conquest games in order to unlock a Champion. Your progress will be displayed to the right of each Champion in this list.
Once you have reached the conditions, you will be able to claim the Champion.