Brienne of Tarth may be a simple knight in a show filled to the brim with highborn rulers and magicians and assassins, but her down-to-Earth nature is a necessary presence in Game of Thrones, providing perspective to royalty like the Stars sisters or Jaime Lannister. Despite having become a crucial part of the show, she was noticeably absent from the action of the Season 8 premiere of GoT, and many viewers may have missed her altogether if they hadn't been paying close attention. Fans who kept their eyes on the backgrounds of certain scenes know where Brienne is in Game of Thrones Season 8, but it's not entirely clear what she's up to despite actress Gwendoline Christie telling Entertainment Weekly that audiences would be getting to see "Brienne going full throttle doing what we all love to watch her do."
What we all love to watch her do, presumably, is take names and get justice. She slaid Stannis Baratheon to avenge the death of Renly, and honored Catelyn Stark's wishes to see that Arya and Sansa be returned safely to Winterfell. She didn't directly bring Arya to Winterfell — but she certainly helped in getting Sansa there.
Since her arrival in Winterfell, she's been sparring with Arya to help her train and is still loyal to Sansa who is now serving as Lady of Winterfell, but what else is there for Brienne to do? Season 8 will likely be wrapping up the show's many plotlines, and it's likely that Brienne may not have been a large part of the season premiere because her individual plot line has pretty much come to an end. However, that doesn't mean she won't play a large role in someone else's story.
In the Season 8 premiere "Winterfell," Brienne is seen amongst the crowd at the titular location, welcoming the arrival of Jon Snow, The Hound, Gendry, and Daenerys Targaryen among others. However, the Winterfell arrival that has the most direct relevance to Brienne is actually Jaime Lannister, who turns up at the very end of the episode to make eyes with Bran, realizing that it's probably bad news that the kid he pushed out of a window is still alive.
Jaime has gone from a blowhard murderer to something resembling an honorable knight, thanks in no small part to having been escorted by Brienne to King's Landing. He's also currently in the most difficult position of his life, fleeing his sister/lover as she sits atop the Iron Throne and strolling into the home of the Starks, which is a pretty bold move for someone's whose family has orchestrated the death of at least three Starks and killed the father of the Dragon Queen, who has made Winterfell a temporary home as she plans her next move and unknowingly sleeps with her nephew.
Brienne is likely the only person at Winterfell whose last name isn't also Lannister who will be willing to vouch for Jaime against all those who, rightfully or otherwise, would wish to see him perish. She's proven herself to be a noble knight, and Season 8 will give her the opportunity to prove that she's made a noble knight of out Jaime as well.