Jedi accept death as natural while the Sith attempt to cheat it. The Sith cannot become true Force Ghosts; only a person who has aligned their spirit with the light side of the Force can do so. Rather, Sith Spirits can cheat death through a technique known as essence transfer.
In both the novel and the comic book version of Revenge of the Sith, the spirit of Qui-Gon Jinn tells Yoda that only light-siders may live on as Force ghosts, and that this is something that dark-siders cannot learn, but this claim is not, in fact, true.
The dangerous level to become a Force spirit was low, but only possible for a rare few Jedi. Qui-Gon Jinn appears to Kenobi for the first time as a Force spirit. The first known Jedi to learn the secret was Qui-Gon Jinn, who learned it from a Shaman of the Whills.
Naturally, only those with a strong affinity to the Force can achieve the posthumous transformation, and the technique was pioneered by Qui-Gon Jinn, though he was killed before perfecting the technique. Fortunately, Yoda picked up Qui-Gon's studies and, along with Kenobi, mastered the Force ghost form proper.
While some Jedi chose to fully subside into the Force after their passing, those who were Force Ghosts could retain their individuality away from it while still being able to bond with its interplanetary magnetic pull. They vow themselves to the Force without giving the Force the power to absorb them completely.
A Jedi must receive special training to become a Force Ghost
Those who died and passed into this netherworld of the Force generally could not affect the material world. However, some individuals can train during their lives to become Force Ghosts, retaining the ability to influence the galaxy after death.
Becoming a Force Ghost was a lost art that Qui-Gon rediscovered. Qui-Gon never completed the process, which is why he only ever appears as voice instead of a full ghost, but he taught it to Yoda and Yoda taught it to Obi-wan.
In material that is now considered Legends, Qui-Gon is even described as a Gray Jedi. Additionally, the term has also been applied to Force-sensitives who aren't affiliated with the Light or Dark side of the Force. Quite literally, these Gray Jedi don't see things in black and white, or light versus dark.
Why didn't Mace Windu showup as a force ghost? Qui Gon died before he could finish his training so he can only appear as a voice and not a physical form. Mace Windu never studied or trained in that aspect of the force.
For years he served as Palpatine's right hand man, but he was ultimately redeemed by the faith of his son, Luke Skywalker. Now a Force Ghost, Anakin continues to act as an agent of balance.
Qui-Gon: I cannot. My training was incomplete. As such, he can only ever materialize as a disembodied voice in the physical world. The other Jedi who have achieved oneness with the Force have disappeared upon death, such as Yoda, Obi-Wan, and Luke.
However, unlike the Sith, who wish to live and hold their power for eternity, immortality through the Force is different. Qui-Gon knew it. He learned it from the Shaman of the Whills, who taught him that becoming a Force Ghost requires a person to be entirely selfless.
Summary. The ending of The Rise of Skywalker sees Leia as a Force ghost, but it's never explained how she was trained to do this after her death. Leia's ability to become a Force ghost isn't far-fetched, as she had three Jedi Masters who could have taught her - Luke, Obi-Wan, and Yoda.
Can a Sith become a Force ghost? No. As far as I know, only a Jedi that is properly attuned with the Force can offer themselves to it and become part/one with it. Qui-gon, Obi-wan, Yoda, Luke, Leia, Anakin, etc.
Anakin's spirit did return to help Ahsoka overcome her past trauma and to make amends with his former Padawan for the terrible things he did as Darth Vader. The last shot of Ahsoka season 1 presents the scene from his perspective, removing all doubt that he was only in Ahsoka's head.
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.
Summary. Qui-Gon Jinn's refusal to conform to the Jedi Order's wishes posed a threat to Palpatine's plan to take over the galaxy. Qui-Gon's strong connection to the Force would have allowed him to detect Palpatine's evil plans much earlier than the rest of the Jedi.
Gray lightsabers are often associated with Jedi who do not fully follow the Jedi Code, but also do not fully embrace the Dark Side. These Jedi are often able to balance the light and dark sides of the Force and use their powers to serve the common good and are less likely to be influenced by the Dark Side.
Force ghosts were limited to those who followed a selfless life, the light side of the Force, of were Jedi. The Sith, and Dark Force users were unable to manifest themselves after death in such a way. And non-force users were unable to return too.
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.
Qui-Gon taught Yoda to become a Force Ghost. We can assume either he or Yoda taught Obi-Wan. Obi-Wan canonically taught Anakin as he was dying after he came back to the light. To become a Force Ghost, confront, accept, and over come/defeat your inner darkness.
These characters include Ahsoka Tano, Kanan Jarrus, Luminara Unduli, Aayla Secura, Mace Windu, and Adi Gallia. While those mentioned have never appeared physically as a Force ghost, their Force connection with Rey can surely be classed as a Force ghost appearance in Star Wars canon.
Some Jedi achieved immortality as Force spirits, their consciousness preserved in the Force. Every living being became one with the Force during the process of dying.
