Goku (Super Saiyan God Super Saiyan) is the SSGSS transformation of Goku that has much more power. He is a DLC character and is included in DLC Pack 3, along with Golden Frieza and Vegeta (Super Saiyan God Super Saiyan)
DLC and Unlocking[]
He's automatically unlocked after DLC Pack 3 is installed and can be a partner, but he isn't a mentor and participates just in the game quests.