Emperor Palpatine carefully concealed the fact he was a Sith Lord, but there were some Star Wars heroes and villains who knew the truth. Darth Sidious was always a schemer, preferring to operate in the shadows rather than stand in the spotlight. When he finally became Emperor in Star Wars: Episode III - Revenge of the Sith, he saw no need to abandon the secrecy that had served him so well for years. The Empire he ruled was characterized by deceit and mistrust, with only the upper echelons knowing the truth about their leader.
The first Star Wars novelization suggests the Emperor managed to deceive the entire galaxy into believing he was a kindly and generous man. The evils of the Empire were blamed on Palpatine's advisers and military leaders, with the public believing the Emperor himself was unaware of what was going on. It was the grandest deceit of all, perpetuated over the course of decades, although he had clearly abandoned it by the time of the Star Wars sequel trilogy. Here are all Palpatine's friends, allies, and enemies who knew the Emperor was a Sith Lord.
Darth Vader
As Palpatine's apprentice, Darth Vader was well aware of his Master's true identity; the revelation that Palpatine was Sidious was instrumental in Anakin Skywalker's fall to the dark side in Star Wars: Episode III - Revenge of the Sith. Darth Vader became the public face of the dark side cult, known to the galaxy as the Dark Lord of the Sith but in reality subservient to his Master. Few knew enough of the Sith to understand there should not be one Sith but two, and any who did probably assumed one of Vader's Inquisitors was actually a Sith Apprentice.
Darth Maul
Although Darth Maul was considered a Sith Apprentice, Palpatine always thought of him as little more than a tool. As such, until Star Wars: The Clone Wars season 7 it was difficult to say just how much Maul knew - including whether he knew Darth Sidious' true identity. Those questions were answered during the Siege of Mandalore, when Maul teased Ahsoka Tano with just enough information to suggest he knew all about Order 66 and Darth Sidious' plans for Anakin Skywalker. Into the Dark Times, Maul planned to use this knowledge as leverage to undermine and destroy Palpatine.
Mas Amedda
A Chagrian politician who served in the Senate from the time of Chancellor Valorum onwards, Mas Amedda was one of Palpatine's closest political allies. He proved himself useful during Palpatine's rise to power, helping him navigate the treacherous waters of galactic politics and witnessing the Sith Lord's ascension to Emperor. He remained implacably loyal to Palpatine over the next few decades, but curiously, he was not approached by the Emperor's droids in order to participate in Operation Cinder after Palpatine's death in Return of the Jedi.
Sly Moore
Sly Moore served as Palpatine's Chief of Staff during his time as Chancellor. Not only was Sly Moore aware of Darth Sidious' true nature, she herself was actually a Force-sensitive, successfully concealing her own dark side powers from the Jedi. The Umbaran woman's past is still something of a mystery, but she remained unflinchingly loyal to the Emperor throughout his reign, and she was still a power-player in Palpatine's government until shortly before Return of the Jedi. She appears to have vanished after the destruction of the Second Death Star, perhaps heading to Exegol.
Grand Moff Tarkin
One of the Empire's foremost military minds, James Luceno's novel Tarkin confirms the Grand Moff became close enough to figure out his master, the Emperor, was actually a Dark Lord of the Sith; he likewise deduced that Darth Vader was Anakin Skywalker. But Tarkin had no real interest in the secrets of the Sith, nor was he at all interested in challenging the Emperor's reign. He proved himself a loyal ally, with his so-called "Tarkin Doctrine" - of using fear to bring the galaxy to heel - fitting in nicely with the Sith philosophy. Tarkin died on the First Death Star.
Grand iral Thrawn
The Chiss strategist Grand iral Thrawn was the only alien to attain such a high military rank in the Imperial Navy, primarily because his knowledge of the Unknown Regions was useful to the Emperor. Timothy Zahn's novels have strongly suggested Thrawn - like Tarkin - swiftly deduced Palpatine's true identity as a Dark Lord of the Sith. Lacking any real interest in the ways of the Jedi and the Sith, Thrawn viewed all this as some sort of intellectual curiosity and disregarded it in order to continue doing his duty in service to the Emperor.
Gallius Rax
Gallius Rax was a Sith cultist who was introduced in Chuck Wendig's Aftermath trilogy. He appears to have numbered among the Emperor's closest advisers, even anticipating his resurrection after the Battle of Endor. Still, for all Rax held Palpatine in high esteem, he did not fully understand the ways of the dark side; he was aware the Emperor sensed a dark presence in the Unknown Regions, most likely the ancient Sith world of Exegol, but he was unable to understand why only Palpatine could detect it. He actually thought the Emperor had gone mad.
Ochi of Bestoon
The Sith assassin Ochi of Bestoon was first mentioned in Star Wars: The Rise of Skywalker, identified as the agent of Palpatine who killed Rey's parents. He's since been woven into the Star Wars timeline, introduced in the current Darth Vader run as an agent of the Emperor who was assigned to target Vader himself for his failure in The Empire Strikes Back. Ochi seems to know far more of the Emperor's secrets than even Darth Vader, with dialogue hinting he was aware of Palpatine's attempts to conquer death and the cultists who were based on Exegol.
Master Yoda
Master Yoda learned the painful truth about Palpatine in Star Wars: Episode III - Revenge of the Sith, and he even engaged the Dark Lord of the Sith in a desperate duel in the Senate chambers. Yoda subsequently retreated to the planet Dagobah to meditate upon the ways of the Force and continue his apprenticeship to the Force Ghost of Qui-Gon Jinn. He was well aware he was not the one who would put the galaxy to rights.
Obi-Wan Kenobi
Like Master Yoda, Obi-Wan Kenobi learned Palpatine's true identity in Star Wars: Episode III - Revenge of the Sith. Where Yoda went after Palpatine, Obi-Wan targeted Darth Vader himself. He subsequently retreated to the desert planet Tatooine to watch over the infant Luke Skywalker, and he came to believe Luke was the true Chosen One who was destined to destroy the Sith. Obi-Wan seems to have largely kept his head down during the time of the Empire, wary of drawing too much attention to himself for fear of exposing Luke to danger, and he died in the first Star Wars film.
It's difficult to say whether or not any of the Rebel leaders knew Palpatine was a Sith Lord
It's difficult to say whether or not any of the Rebel leaders knew Palpatine was a Sith Lord. Although Bail Organa heard Yoda and Obi-Wan talking after their fateful battles with Darth Sidious and Darth Vader, he was never actually told. He may well have put the pieces together, however.
