When a player challenges you in multiplayer mode, your defense team is automatically assigned to fight him. You cannot use your defense team on your attack team or use it in dungeons. You can, however, change your defense team’s characters by replacing a character from your defense team with another. Now that the old character is free, you can assign him to your attack team.
To change your defense team characters, click PvP and then tap on the “Change Your Team” button. Next, select the “Defense Team” tab and tap the “Change” button underneath each character to replace it with another.