Depending on the rules for the match, you can have multiple characters on your team in Dodgeball Academia. While movement and commands will all be acted out by each character under your control, there is a primary character who stands at the front.
To switch which character is your first spot, simply pass the ball by pressing RT or the E/U key. You can also switch when you don't have a ball. Doing so will switch your primary character to the back row and bump the character you passed to up to the front. Some matches will have one character on the court and others on the sidelines. Passing the ball will switch the character on the court with one on the sidelines.
This is important as each character has hit points and being at the front of your team will cause them to bear the brunt of the damage. Switching between your characters will also let you use their Balltimate Move. Each character also has a different way of throwing the ball and countering.
Knowing when to switch characters is important and something you should learn as you play. Keeping all of your teammates on their feet is crucial to winning in Dodgeball Academia.
For more information on Dodgeball Academia, check out Can you hit opponents behind you in Dodgeball Academia and How to change your team in Dodgeball Academia on Pro Game Guides.