WARNING: Star Wars: The Rise of Skywalker.
A flashback scene in Leia was the sister of Luke Skywalker and daughter of Darth Vader. Like her brother and father, The Empire Strikes Back and Return of the Jedi revealed that she was Force-sensitive to some degree. In both of those films, Leia is shown to have the ability to sense Luke's whereabouts, even before she learns of the bond they have.
When Carrie Fisher returned to the role for the new Disney sequel trilogy, the films continued to explore Leia's connection to the Force. After all, she was the other hope that Yoda was referring to in The Empire Strikes Back who could save the galaxy. Leia concentrated on politics instead of becoming a Jedi, but her Force abilities were still on display. She felt the death of Han Solo in Leia used the Force to fly back to the ship that she was blasted from. If viewers still had questions about Leia's ability, The Last Jedi made it clear that she was strong in the Force. The only question that remained was how strong she was and when (or if) she received any training.
Originally, only about eight minutes of usable footage though, meaning Leia's role had to be reduced to some degree. While Lucasfilm and Abrams clearly stated that they weren't going to digitally recreate Fisher to bring Leia back in the movie, they did use a de-aged version of her. It is in this scene that The Rise of Skywalker reveals Leia became a Jedi.
Leia Trained With Luke After Return of the Jedi
In The Rise of Skywalker, it is revealed that Luke trained Leia in the Force. This flashback sequence isn't the first training session between Leia and Luke though, but rather the last night of her training according to Luke. The scene takes place after Return of the Jedi, but the exact setting of the scene is unclear. It shows two individuals wearing blinding helmets that conceal their faces and fighting through the forest with lightsabers. Luke battles with his green lightsaber and Leia wields her own blue lightsaber. The fight doesn't show Leia using any new Force powers, but it does demonstrate her ability in combat as she bests Luke in their fight. Clearly, Luke has taught Leia well and she was well on her way to being a true Jedi.
Leia being trained in the Force by Luke is something that Star Wars fans have seen before in the now non-canon Legends books and comics, but it is also something Mark Hamill said George Lucas Leia who trains Rey in The Rise of Skywalker. They've set up a training course on the base planet of the Resistance and Rey even refers to Leia as Master. Rey isn't just taking Leia's advice because of her bloodline, but rather because she knows what she's talking about.
Why Leia Gave Up Jedi Training
Even though she is called a Master, it is important to note that Leia does not appear to complete her Jedi training to take on this title - unless she became one when Luke was attempting to rebuild the Jedi Order. In any case, Leia decided to stop her training before taking on the duties of a Jedi because she sensed that it would not end well. She felt that Ben Solo would die if she continued her Jedi training. To try and prevent this from happening, Leia laid down her lightsaber and gave up her training. Instead, she focused on politics and trying to raise her son. Unfortunately for Leia, the future she saw still came to be even though she was no longer training.
The idea of Leia stopping her Jedi training is something that has already been confirmed as canon, but Star Wars: The Rise of Skywalker adds new details to her decision. In Star Wars: The Last Jedi - The Visual Dictionary, Pablo Hidalgo wrote that Leia was going to be Luke's first student but "she ultimately decided that the best path for her to serve the galaxy left no room for the extended isolation of Jedi training." It turns out that Leia was Luke's first student after all, but her decision to give up the Jedi training is two-fold. Leia wanted to protect the galaxy as a whole but also try and save her son.
Leia's Lightsaber & Why It's Important
Once Leia decided to stop her Jedi training, that meant laying down her own lightsaber for good. Star Wars: The Rise of Skywalker doesn't show when or how Leia constructs her weapon - likely a hole that a novel or comic book will explore - but this is a process that typically happens early on in the training of a Jedi Padawan. Leia's lightsaber remained in Luke's possession after she quit because she believed that one day it would be picked up again. For that reason, Luke kept it with him and hid it away on Ahch-To. But, after telling Rey about Leia's Jedi training, he shows Rey where she can find it so she can use it in her quest to defeat Palpatine.
It's for that purpose that Leia's lightsaber winds up playing a critical role in the finale of The Rise of Skywalker. After loaning Luke's repaired blue lightsaber to Ben to defeat the Knights of Ren, Rey uses Leia's lightsaber to fend off some of Palpatine's followers. This fulfills Leia's belief that someone else would pick up her lightsaber when the time was right. In the end, she uses both of the lightsabers to deflect Palpatine's Force lightning back to him and kill him, once and for all. It is Rey's power and the weapons of both of her Jedi Masters that enables her to save the galaxy.
But, neither of these lightsabers are inherently Rey's, so she who takes on the Skywalker surname - sees both Luke and Leia in Force ghost form, showing the extent of what Leia learned from her brother before giving up her Jedi training.
