While on the battlefield going at it against an opponent with one of your very own fighters, you will also have the option of getting assists from your other teammates or switching to them at will. In order to call for an assist, simply press the designated Button that you have for that given character. This will cause the fighter to quickly come into the battle, using an attack against the foe. In order to switch into another of your fighters, simply hold down said same Assist Button. But do keep in mind that after Switching or using an Assist, there will be a short period where you can do neither, forcing your fighter on the battlefield to have to go at it alone.