Though it's possible that he simply never had the knowledge to become a Force ghost – Star Wars: The Clone Wars and Obi-Wan Kenobi proved that retaining one's identity and consciousness after death was no easy task – it's equally possible that Kylo Ren could still return as a Force ghost, or as something else entirely, ...
Originally Answered: Why doesn't Ben Solo appear as Force Ghost? Because he's now literally a part of Rey. Earlier in the movie, Rey healed that snake thing and said she transferred part of her life force to it. That was foreshadowing that when Rey died it look all of Ben's life force to grind her back.
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.
Force Ghosts have become part of the Star Wars franchise's lore, expanded upon in movies, numerous canon tie-ins, and even the Obi-Wan Kenobi Disney+ TV show. Some of the greatest Jedi returned as Force Ghosts: Qui-Gon Jinn, Yoda, Anakin Skywalker, Luke Skywalker, and Leia Organa.
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.
Why Luke Believed Anakin's Force Ghost Stopped Appearing to Him - Star Wars Explained
Why is Qui-Gon Jinn not a Force ghost?
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. His training remained incomplete at the time of his death, so he could only manifest as a disembodied voice.
The closest that Star Wars canon has come to naming who trained Yoda was revealing that Jedi Master Qui-Gon Ginn was the one who taught Yoda to become a Force Ghost and attain immortality.
Also, the Sith Lord Ajunta Pall lived on through the Force in such a manner as a Force ghost, though it is relatively unknown how or why he remained. However, it can be speculated that his extreme rage and anger made his presence linger.
The crossguard blades allow Ren's saber to operate safely – his weapon is constructed around a dangerously flawed kyber crystal, which would be overloaded if not for the lateral plasma vents that create the quillons.
The fight ends with Braga convincing Darth Sajar to abandon the Dark Side and instead become a part of the Jedi Order. Since then, Sajar had become Braga's Padawan and began his studies of the Light Side.
Despite his origins, Snoke was not a Sith, but was used as a tool by them to groom and mold Ben Solo, the eventual Kylo Ren, into an heir worthy of Darth Vader's legacy, testing his capacity to inherit the Sith legacy as a whole and rule the galaxy as the new Emperor.
It seems that with his last ounce of strength, Ben gave what was remaining of his life energy to save Rey. This was his last sacrifice, effectively acting as redemption to return him to the light side of the Force. That is why his body vanished, as Jedi like Luke Skywalker and Obi-Wan Kenobi and Leia did before him.
during the battle, Ben lets Ghostfreak possess him so that his DNA is re-transferred to the Codon Stream. Ben then changes into Ghostfreak to control Ghostfreak but Ghostfreak possesses him and knocks out his friends. He then challenges Vilgax who weakens him with the light from his sword.
Ben lost Ghostfreak after he escaped the Omnitrix and became an antagonist, eventually being the main villain of Season 3. However, at the S3 finale, Ben actually regains Ghostfreak as an alien once again after Ghostfreak touches the Omnitrix, adding him back into the system.
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.
According to Imperial Estimates in Star Wars Inside #87, only about 100 of the Jedi survived Order 66. Most were subsequently hunted down and killed in the years following the Great Jedi Purge.
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.
We know some of Yoda's padawans quite well. The most obvious one was Luke Skywalker (who died stupidly). Another famous one is Count Dooku (who died at the hands of Anakin Skywalker).
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.
Legend had it that Yoda—a Jedi who became Grand Master—was trained by N'Kata Del Gormo. A Force-sensitive Hysalrian, N'Kata Del Gormo was trained in the ways of the Force and achieved the rank of Master within the Jedi Order.
