Yes, there were Sith Purebloods who became Jedi, most notably Lord Praven in Star Wars: The Old Republic (SWTOR) lore, who turned from Sith Warrior to Jedi after being shown mercy, and ancient Je'daii like Sek'nos Rath, precursors to the Jedi Order, who were Sith species but studied the Force's balance on Tython. While Sith Purebloods have a strong natural affinity for the Dark Side, individuals from the species can choose the light, though it's considered a rare path.
One of the Darths between Bane and Sideous tried to sabotage Banes plans by destroying all of the Sith knowledge but his apprentice killed him and continued the line that eventually reached Sideous. Galen marek went from being a sith assasin to a self-identifying jedi.
As a Sith, Praven was very strong in the dark side of the Force, though he felt stronger with the light side as a Jedi. He wielded a single red-bladed lightsaber as both a Sith and a Jedi, and he was skilled in the use of a wide variety of Force powers, including Force lightning, Force blasts, and Force paralysis.
98% of Imperial citizens under Vitiate's Empire had Sith ancestry. By the end of the New Sith Wars, the Sith Purebloods were effectively extinct, save perhaps those of the Lost Tribe of Sith, who were eventually wiped out by the Kesheri Sith.
Sifo-Dyas was a human male Jedi Master from Minashee who commissioned the Kaminoans' creation of the Grand Army of the Republic over a decade before the Clone Wars.
No, Darth Plagueis is not Anakin Skywalker's biological father, but there's a strong theory (especially in Star Wars Legends and early drafts) that his experiments with midi-chlorians to create life led the Force itself to create Anakin, the Chosen One, as a reaction, making Plagueis the "unintentional father" of the prophecy. In current canon, Plagueis is a legend Palpatine uses to lure Anakin, and the exact nature of Anakin's conception by the Force remains a mystery, though he had no conventional father.
Some members of the Sith species were taken from their Korriban homeworld in this time, to eventually become some of the first Je'daii.
In Legends, probably the most unusual species to join the Jedi were the Iron Knights - a silicon based species that looked like shards of crystal. To function in galactic society and as Jedi, they used modified droids to serve as humanoid bodies.
Baylan and Shin Hati are Dark Jedi, not Sith, as they adhere to certain practices of the fallen Jedi Order despite using the dark side. Baylan and Shin have visual parallels to Darth Vader and Darth Maul but are unique dark-side villains with their orange-red lightsabers.
Darth Maul was stronger than Obi Wan, he proved that when he killed Qui-Gon but he was over primed and traded being a showman for his life. Another contender for the weakest Sith could be Count Dooku, or has he's also known Darth Tyranus.
The clone commando RC-1136, known as Darman Skirata, got Jedi Knight Etain Tur-Mukan pregnant during the Clone Wars, resulting in their son, Venku Skirata, a Force-sensitive child born outside the Jedi Code's restrictions. Their forbidden relationship and child's birth were detailed in the Republic Commando novels, highlighting the complexities of clone troopers' development and the strictures of the Jedi Order, as Etain was killed shortly after Order 66 while protecting her family.
The Lost Twenty, also known as the The Lost, was a name given to the group of twenty Jedi Masters who left the Jedi Order throughout its history. According to Yula Braylon of the Jedi High Council, the Lost Twenty became disillusioned with the Jedi way of life.
The most famous case of a Jedi being capable of bringing another Force users who had fallen to the Dark Side of the Force back to the Light is of course Luke Skywalker, whose compassion, incorruptible nature and pure love for his father brought Anakin back from the dark side and allowed him to bring back the balance to ...
Qimir has already been calling himself Darth Venamis, and is ready to officially be a Sith Lord Apprentice, but, is also already conspiring to get a pupil of his own, & eventually kill his own master as well.
Asajj Ventress. A former Jedi Padawan turned assassin, Asajj Ventress was trained in the ways of the dark side by Count Dooku. For many years, she yearned to be considered a true Sith, secretly acting as Dooku's apprentice.
San Tekka was a member of the Church of the Force and a believer in the ideals of the Jedi Knights. He believed that the Jedi were essential to the balance of the Force and helped the Resistance find the vanished Jedi Master Luke Skywalker, unable to stand idly by as despair and darkness grew in the galaxy.
The answer, revealed in Episode IX: The Rise of Skywalker, rewrote much of what we thought we knew. Rey is the granddaughter of Emperor Palpatine, also known as Darth Sidious or Sheev Palpatine—not through a traditional lineage, but through a failed clone named Dathan, Rey's father.
Ahsoka Tano from Star Wars: The Clone Wars can also technically be called a Gray Jedi, due to her forsaking the ways of the Jedi, but still following a path of good. However, neither of these two ever trained to use the dark side of the Force, so they are arguably not "true" Gray Jedi.
In Star Wars lore, Order 37 was a grim Clone Trooper contingency plan to capture a wanted individual (often a Jedi) by taking a civilian population hostage, locking down the area, and threatening mass execution if the target wasn't surrendered, making it a horrific tactic used by the Empire to force Jedi compliance. It was considered more brutal than Order 66 because it targeted civilians directly, forcing Jedi to often turn themselves in to save innocent lives, as seen on planets like Bellassa.
Tràkata is a rare and cunning lightsaber technique built on deception and misdirection. Jedi like Kao Cen Darach and Cal Kestis used it to gain surprise advantages in battle, while Sith like Qimir mastered its lethal precision.
20 Most Powerful Jedi in Star Wars, Ranked
Tarre Vizsla was a Force-sensitive human male who, was inducted into the Jedi Order around 1050 BBY, becoming the first Mandalorian to do so. Tarre eventually became a famous Jedi Knight. Vizsla created the Darksaber during his time as a Jedi.
Temiri Blagg was a Human Jedi Master of the New Jedi Order.
Sith apprentices were analogous to Jedi Padawans, and the relationship between Sith Master and apprentice was the analog to Jedi Knight and Padawan.