Whis is the angel of Universe 7, Beerus's attendant and, as we learn at the end of his first adventure, his martial arts master too. He debuts alongside him in the 2013 film Battle of Gods and appears in every saga of the Dragon Ball Super anime. Whis's profile tells you who he is; here we follow him saga by saga, from smiling assistant to the teacher who brings Goku and Vegeta up to the level of the gods.
Dragon Ball Super
Battle of Gods Saga (episodes 1-18)
In the Battle of Gods Saga he is the one who wakes Beerus and joins him in the search for the Super Saiyan God: like every angel, he follows his God of Destruction almost everywhere, watches over him and, when needed, keeps him in check. At Bulma's party he falls in love with Earth food, watches the god's fight with Goku and in the end carries a sleeping Beerus home. The film reveals his secret: he is Beerus's master and stronger than him. From here the series turns him into a trainer. In episode 16 Vegeta finds out that Bulma has been having lunch with him to keep Beerus happy and wins him over as a teacher with her instant noodles; Goku is accepted soon after. In episode 18 the two Saiyans discover his method: chores on Beerus's planet first, then a sparring test in which neither of them manages to land a single hit.
Resurrection 'F' Saga (episodes 19-27)
In the Resurrection 'F' Saga Bulma contacts him to call Goku and Vegeta back to an Earth threatened by Frieza. On the way he runs into his sister Vados and Champa, then reaches the battlefield with Beerus. When Frieza blows up the planet, in episode 27 Whis shields Goku and the others, then turns back time a couple of minutes to give Goku one last chance.
Universe 6 Saga (episodes 28-46)
The Universe 6 Saga reveals his family: in episode 28 Vados introduces herself as his older sister, and Whis explains to Goku and Vegeta that there are eleven other universes besides theirs. Once the tournament is over, in episode 41 he summons Super Shenron for Beerus's wish.
Future Trunks Saga (episodes 47-76)
In the Future Trunks Saga his power over time is decisive again. In episode 59, in Universe 10, Zamasu kills his master Gowasu and Whis rewinds time to save him, before Beerus wipes out the apprentice. In the finale, in episode 67, we learn that the Grand Minister is his father, and it is Whis who suggests that Trunks and Mai move on to a new future.
Universe Survival Saga (episodes 77-131)
In the Universe Survival Saga he uses his powers over time once more: in episode 83 he speeds up Bulla's birth so that Vegeta can join the Tournament of Power. During the tournament he comments on Goku's Ultra Instinct, the ability every angel has mastered and that his training had hinted at; in episode 116 Vegeta realizes that this was exactly Whis's lesson. In the end, in episode 131, he brings Frieza back to life at Beerus's request.
Movies
Besides Battle of Gods, Whis is central to Resurrection 'F', where he trains Goku and Vegeta on Beerus's planet and turns back time by three minutes. In Super Hero he is still on Beerus's planet while Goku, Vegeta and Broly train, but Beerus's ice cream cup ends up on his staff and blocks any contact with Earth.
Did you know? Masakazu Morita, Whis's Japanese voice, had already voiced Tarble, Vegeta's brother, in the 2008 special.
Our take
Saga after saga, Whis is the character who makes Super work without ever stealing the spotlight. He is the teacher Goku and Vegeta were missing, and also the series' most powerful storytelling tool: he rewinds time against Frieza, saves Gowasu, brings forward a birth, revives an enemy.
That is exactly why the series uses him sparingly. His strength nearly always stays off screen, described by others, and his calm smile says more than any fight: as long as Whis is not worried, nobody really needs to be.
For the god he serves, read Beerus saga by saga; for his students there are Vegeta saga by saga and Goku's profile.