In the finale for "Foundation" season 3, we discovered that Bayta (Synnøve Karlsen) was The Mule in disguise the whole time. After weeks of compelling but predictable "big bad" villain action from Pilou Asbæk's red herring character (who just ended up being a space pirate caught up in the real Mule's plans), we found out that the sweet and innocent Bayta was actually the mentalic pulling the strings behind the scenes the whole time.
It was a stunning reveal -- particularly because it runs polar opposite to Asimov's books. In the original version there is still a mislead, but it isn't Bayta who ends up being the Mule: it's the musically-minded jester Magnifico Giganticus. Magnifico is born on a planet called Gaia, which will be very important later in the show. He rebels against his people and runs off to pursue his own galaxy-conquering goals. (If that sounds odd, it's because the show has also changed the Mule's backstory.)
Bayta is still intimately involved in the Mule narrative in the books, though. She's the key character who bonds with Magnifico/The Mule, who loves her and doesn't want to mess with her mind. This "untampered with" mental condition ends up allowing Bayta to become the only chance of stopping the conqueror from discovering where the Second Foundation is hiding. Eventually, the First Speaker of the Second Foundation gets involved and defeats the Mule, but Bayta remains a good guy throughout the story, begging the question: why did she become the primary antagonist in the show?