Your Middle is the second character on your Team. Usually, this slot is for a fighter that is equally as great as your Point that you can switch to in order not to lose any ground. However, your Middle could also be used to assist your Point, helping by covering one of your Point's weaknesses or by extending your Point's combo moves.
One example of this is having Goku as your Middle with Teen Gohan as your Point, as Goku can assist Teen Gohan get closer to foes with his Kamehameha assist. Another example is having Adult Gohan as your Middle with Frieza as your Point, as Adult Gohan can assist Frieza by getting foes off of Frieza with his Dragon Flight assist.