A Champion will be selected by default when the battle starts.
They will move to the tapped area while attacking the opponent's unit and will focus attack the unit when you tap the opponent's unit directly.
You will need to be careful when you attack the enemy unit directly, since your Champion will follow your order and even move into the enemy base to attack.

If a unit Card is selected, you can touch the Champion icon on the bottom right and then control the Champion again!