When you fulfill certain conditions in Story Mode, you will be able to use additional characters not officially part of the story. Once unlocked, you will be able to use these characters in any of the Story Mode campaigns.

Unlockable	How to Unlock
Goku (SSGSS)	Reach Lv. 40 with Goku (Super Saiyan)
Vegeta (SSGSS)	Reach Lv. 40 with Vegeta (Super Saiyan)