There's a third order out there in the Star Wars universe, even if it has since been relegated to the non-Disney canon "Legends" materials. They are the Jensaarai, an order of Force users who embody certain philosophies and characteristics of both Jedi and Sith, but follow their own path.
While the term itself made a few appearances in the Star Wars Legends continuity, the Gray Jedi have yet to be officially referenced in canon. It's worth noting that the Lucasfilm Story Group has never embraced the term, believing that Gray Jedi can't be canon for misunderstanding balance as described by George Lucas.
Basically, everything except those items marked with an "Infinity" logo (i.e. the Star Wars Tales comics) is considered canon. There is a hierarchy -- the movies, novelizations, radio dramas come first. Then everything else. If something in a novelization contradicts the movies, then we defer to the movies.
Is Cal Kestis canon in Star Wars? Yes, Cal Kestis canon in Star Wars. Both the Star Wars Jedi games, Fallen Order and Survivor, are canon. Both games contain explicit references to events of the film series and its companion material such as Order 66.
Canon is everything that has been made post-Disney buyout as well as all of the movies and the Clone Wars show. This includes any books, comics, and games released after 2012. These are the "actual" events of the Star Wars universe, whereas Legends is its own story that concluded when Disney bought Lucasfilm.
Why The Jensaarai “Grey Jedi” Were Such DEADLY Fighters - Star Wars Explained
Is Darth Plagueis canon?
It is very unfortunate that the story is part of Legends instead of canon, however, a lot of it has indeed made it back into canon thanks to people like the original author starwars.wikia.com/wiki/James_Luceno .
Cal Kestis and Starkiller come from vastly different places, but they may be two sides of the same coin. One was forged by Vader while the other one was forged by rebellion, but they both ultimately do what is right in the end.
During his training to become a Jedi, Kestis traveled with Grand Master Yoda to a Jedi Temple located on the ice planet Ilum and participated in The Gathering, a rite of passage where Jedi younglings retrieved a kyber crystal to build their lightsabers.
Ok so let's get into it Mace Windu survives, The fall but he has lost his weapon and is servilely wounded in now hostile territory. This is why I believe he would go into hiding for some time thoughout the early years of The Galactic Empire.
Though seemingly killed by Obi-Wan Kenobi at the end of the film, Darth Maul returned in the 2008 animated series Star Wars: The Clone Wars (voiced by Sam Witwer).
An iconic video game character became canon with Star Wars Episode IX. Introduced in the classic Star Wars: Knights of the Old Republic games, Darth Nihilus was a wound in the Force, more of a malicious presence than flesh and blood.
The term “gray Jedi” in Star Wars is somewhat of a misnomer, considering that the definition refers to one who is practically non-Jedi. That being said, quite a few Jedi can be placed into this category. But Mace Windu identified as a Jedi despite his possible inclusion in the "gray" category.
Who Do Fans Consider Gray Jedi? Aside from Qui-Gon Jinn and Ahsoka Tano, other characters have been dubbed Gray Jedi by fans, even though the term isn't canon. For instance, Kylo Ren — a.k.a. Ben Solo — is a Force-user who taps into both the Dark and Light sides of the Force.
Characters like Cade Skywalker, Asajj Ventress, Quinlan Vos, Kreia, Revan, Kanan Jarrus, Ezra Bridger, and Ahsoka Tano all demonstrate different aspects of being Gray Jedi.
Within the franchise, Cal is a former Jedi Padawan and survivor of the Great Jedi Purge who, during the early reign of the Galactic Empire, lives in seclusion on the planet Bracca until an incident forces him to reveal his Force powers.
Darth Vader was Starkiller's master. He gave Starkiller tasks and missions to complete, and was hoping to use him to overthrow Darth Sidious and take over the Empire. But he was NOT Starkiller's friend or father figure. He was abusive, starving Starkiller and forcing him to fight when starved or...
Some time after this quest Cal was rifted into the Multiverse Merged universe where he met Luke Skywalker and formed a New Jedi Order with him. Although recent hardships have strained their relationship, Cal and Luke are still trying to restore the Order, now much better than it was before.
Regardless of how much Vader remembered or cared about Cal prior to their encounter in Star Wars Jedi: Fallen Order, he is certainly aware of Cal's presence and potential now.
It's worth noting that there is something of a conflict in terms of the timeline here as The Mandalorian takes place after Return of the Jedi. Cal's fate throughout the original trilogy has yet to be revealed, and the character would need to be aged up for any sort of cameo appearance in the Disney+ series.
That didn't stop fans speculating on whether Snoke was really Darth Plagueis or Anakin Skywalker's evil twin or something else entirely. It didn't turn out to be anything as grand when The Rise of Skywalker revealed that, actually, Emperor Palpatine was in charge all along.
However, given that this confirmation was never made into the movie, nor was it ever revisited, the idea that Darth Plagueis created Anakin remains just a theory. The same applies to whether Palpatine created Anakin, which has never been confirmed by the Star Wars canon.
Darth Plagueis traveled the galaxy with his master, masquerading as Rugess Nome's young accountant while the two carried out the Sith plan of spreading discontent to ripen the galaxy for their rule.
