Whis is a constant neutral character in the Dragon Ball Super anime. He is always polite and smiles at anyone regardless of the person’s status. Although food is one of the few things that makes him show his real character more, he can also be seen smiling when he witnessed a good fight. This is true especially if that fight involves one of his trainees. Goku’s Ultra Instinct even made him blush like that.
The latest extended preview of Dragon Ball Super episode 129 shows Whis in a kinda different shade of happiness. After seeing Goku breaking his limit once again, Whis was seen smiling a rather knowing smile. Perhaps he knew back then that he succeeded in turning a mortal into a God-like creature.
This smile is a rather out of character for Whis, as he rarely does a sinister smile like that. He only does that when he knows something that nobody does. This smile is supposed to happen during Goku’s transformation from Ultra Instinct Omen to Mastered Ultra Instinct. Because we have only seen the visual so far, we still don’t have a clue about the Mastered Ultra Instinct ability.
Whis’ reaction could mean so many things. It can mean Goku finally owned Ultra Instinct. In his first transformation, the trigger for this is his absorption of the energy from the Spirit Bomb. The second time he transformed, it was Kefla’s enormous power that awakened the transformation. The third and current transformation is triggered because of Jiren’s overwhelming power.
The Mastered Ultra Instinct will probably be because of Jiren’s overwhelming power, even to Ultra Instinct Omen. There is a scene in the preview where Jiren shielded himself against UI Goku’s Kamehameha. After that, Jiren gave it back to Goku, the Saiyan received a direct hit. As Mastered Ultra Instinct appears, Whis realized that Goku is now somehow on his level. If he wills it, he can now be a God of Destruction in the making, just like Toppo. Even more, Goku would be more powerful than the U11 GoD candidate; and Whis smiles because he knows that.