OLD KAI (ROU KAIOSHIN) SPOILER!

The ancient, deeply eccentric deity who accidentally got sealed inside the Z-Sword because Beerus was having a petty argument with him millions of years ago. He is the entire reason Gohan achieves his "Ultimate/Mystic" state without needing to turn his hair yellow, bypassing the entire Super Saiyan power scale. Furthermore, he sacrifices his literal remaining life force to give Goku his soul back for the final battle against Buu. Leaving him out makes the endgame lore look entirely incomplete.