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The story of Kylo Ren from the Star Wars sequel trilogy.
Ren was born as Ben Solo, the son of Han Solo and Leia Organa, a year after Return of the Jedi, in which his parents and uncle Luke defeated the Galactic Empire and its leader Emperor Palpatine. He proved to be sensitive to the Force like his mother and uncle.
Fearing that Ben was too much like his maternal grandfather Darth Vader, Leia sent him to Luke to teach him about the ways of the Force and become a Jedi. However, Luke sensed a force of darkness inside Ben and contemplated on killing him, but cannot bear himself to do so because it would break both Leia and Han's hearts. However, Ben soon learned of Luke's intentions and took this as a sign of betrayal, fearing that he'll never be as powerful as he wanted to be. This insecurity is what allowed Snoke, the leader of the evil First Order organization, to corrupt Ben to the Dark Side. Sensing that Ben was on the edge of being turned to the Dark Side, Luke decided to go to Kylo's hut - located away from the Jedi temple - and to talk to him about it. He went into the hut where he found Ben sleeping. To his horror, Luke sensed the Dark Side in Ben and realized that he had already been corrupted by Snoke.
For an instant, Luke thought that killing Ben would end the danger and he drew his lightsaber but immediately ignored the thought. However, it was too late. Waking up at the sound of the lightsaber, Ben saw his armed uncle standing above him. Furious and hurt by Luke's betrayal, Kylo went into a fit of rage and brought down the entire hut with his Jedi powers, burying Luke underneath. He then set out and murdered most of Luke's pupils, with a few joining him on the Dark Side. After torching the Jedi temple, Ben and his allies left and when Luke finally freed himself from under the hut, he only found death and destruction among him.
Ben adopted the new name "Kylo Ren" and formed the Knights of Ren out of Luke's former disciples who had joined him on the Dark Side. He would also don the uniform of a Sith, with a black cloak and a mask (resembling Darth Revan's Sith attire from the Legends series) and officially join the First Order as Snoke's disciple. He also developed a rivalry with General Hux for Snoke's appreciation.
Star Wars: Bloodline
The reason for Ben becoming evil was not just Snoke's corruption and Luke's supposed betrayal, but also his parents' tumultuous relationship, given Han's career as a smuggler and Leia's role as a political figure. Six years prior to events of The Force Awakens, Leia's political enemy Carise Sindian exposed her parentage to the public, which is likely the reason how Ben learned that he was the grandson of Darth Vader.
Star Wars: Episode VII - The Force Awakens
Knowing that somewhere on the planet Jakku there is a map leading the way to Luke, Kylo arrives on the planet. His stormtroopers quickly round up all citizens of the nearby village, including village elder Lor San Tekka, who gives the map to Resistance pilot Poe Dameron before Kylo arrives. Poe witnesses the chaos the troops cause and gives the map to his droid BB-8, telling him to get away as far as possible. Kylo interrogates San Tekka, who recognizes him as Han's son and tells him that he cannot run away from the family he was born into. Kylo then murders the man with his lightsaber and orders Captain Phasma to kill all the captured villagers before putting the village off the map.
While the stormtroopers carry out the gruesome task, one of them, Finn, does not participate in the slaughter. Poe, witnessing the murder from a hiding spot, fires his blaster at Kylo, but Kylo stops the laser blast mid-air, freezing it. Kylo then uses the Force to overpower Poe and has the stormtroopers take him captive. After Poe is brought onboard, Kylo leaves the scene and frees the frozen laser blast.
Aboard a Star Destroyer, Kylo interrogates Poe and questions the location of the map. Poe's unwillingness to cooperate prompts Kylo to torture him using the Force to the point where he gives in and confesses the location. He sends his troops back to Jakku to find BB-8, who has already met the scavenger Rey. Poe is then saved by Finn, who has chosen to desert the First Order, and escapes with him to Jakku. After being told about Poe's escape, Kylo furiously demolishes the ship's interior with his lightsaber. Kylo later meets with Snoke, who tells himthat he soon will have to face his father. Kylo replies that, by Snoke's training, he is prepared, but Snoke nonetheless calls it the hardest test Kylo has to face.
Later, after Finn and Rey have met Han and Chewbacca, Kylo tracks the signal of the Millennium Falcon to Takodana. While his soldiers attack the building Finn and Han are in, Kylo tracks down Rey in the forest. Upon finding her, Kylo demands to know the location of BB-8, but after realizing that Rey has already seen the map, he takes her captive, planning to get the information out of her. Kylo tells his troops to retreat, as he now deems BB-8 unimportant.Kylo brings Rey to Starkiller Base. When Rey refuses to talk to a man hidden behind a mask, Kylo takes off his mask, revealing how young he really is. When she refuses to tell him Luke's location, Kylo tries to break her spirit by using the Force, but is shocked when Rey blocks him out of her head, instead reading his mind and revealing his greatest fear: That he will never be as powerful and evil as Darth Vader. Disturbed by the fact Rey is actually strong with the Force, Kylo leaves the room, leaving Rey behind with a stormtrooper guard. However, Rey uses a Force trick on the stormtrooper to force him to free her. When Ren returns to see Rey's chair empty, he flies into a rage and destroys the room's interior with his lightsaber while calling the guards, prompting two nearby stormtroopers to leave the scene before Kylo can harm them.
After General Hux's speech, Kylo witnesses the destruction of the New Republic's capital planet as well as the Republic fleet by Starkiller Base. When the rebels witness the power of Starkiller Base, they plan to destroy the facility before the First Order can destroy them with it. Han, Chewbacca, and Finn plan to infiltrate the base to turn off its shield generators as well as to save Rey. Before they leave, Leia tells Han to try to get their son back, which he promises. At the base, they deactivate the generators and rig parts of the facility with explosives. Kylo, who is present on the base, senses his father's presence and tries to find him.
Han instead sees Kylo, who is just crossing a bridge; he then calls him by his birth name, Ben, and approaches his son. Although Kylo states that Ben Solo was weak and therefore eliminated by Kylo Ren, Han tells him that Snoke corrupted him and that he still has the choice to change. He also tells his son to come home, as his mother misses him. This touches Kylo, who admits that he is torn. Knowing that he would feel drawn to his family as long as his father is there, Kylo begs Han to help him in executing the choice he has made. Han promises and Kylo slowly hands him his lightsaber, intending for his father to kill him to end the pain. However, it turns out to be a ruse as Kylo activates his lightsaber and thrusts it into his father's chest, thanking him for making this choice possible, as without his father, his ties to the Light Side are forever severed. Han's deceased body falls into the abyss. Chewbacca, furious about his friend's death, activates the explosives and shoots at Ren, hitting him and dropping him to the ground.
Kylo sees Rey and Finn leave the building, then pursues them into the frozen woods outside of Starkiller Base. Rey tries to attack Kylo but he uses the Force to throw her far off into the trees, knocking her out. While Kylo screams at Finn that he is a traitor, Finn calls Kylo a monster and activates Luke's lightsaber. The two engage in a furious swordfight in which Kylo gets the upper hand by ramming the crossguard of his lightsaber into Finn's shoulder. Though both of them are hit and injured multiple times, Kylo in the end defeats Finn and slices his back, causing Finn to drop the lightsaber and fall to the ground.
Kylo then tries to use the Force to get his grandfather's lightsaber, but is shocked when the lightsaber flies right past him into the hands of Rey, who now attacks Kylo to avenge both Han and Finn. As a climactic fight progresses, the rebels manage to destroy Starkiller Base, which results in a massive earthquake which creates deep rifts in the surrounding landscape; one of these chasms opens right behind Kylo and Rey.
Kylo drives Rey towards the edge of the rift. While pressing his lightsaber against hers, Kylo offers to train Rey. This, however, causes Rey to realize her potential and, with the Force, she drives Kylo back, hitting him multiple times with her lightsaber and eventually slicing him across the face. Badly injured Kylo drops to the ground but before Rey can do more, a rift opens right between them, separating Rey from Kylo. Chewbacca then steers the Millennium Falcon towards Rey and she boards the ship with Finn's comatose body, leaving Kylo behind on the imploding planet. Shortly before the ultimate destruction of the planet, Snoke gives Hux the order to evacuate Starkiller Base and to bring Kylo before him so that Kylo's training can be completed.
Though Kylo is only last seen when a rift separates him from Rey, he fails to prevent the return of the Jedi when Rey finally finds Luke and tries to convince the last surviving Jedi Master in the galaxy to re-build the Jedi Order once more with her aid.
Star Wars: Episode VIII - The Last Jedi
Following the destruction of Starkiller Base, Kylo feels that he is growing weaker ever since he killed his father; even a disappointed Snoke compares Kylo unfavorably to Vader by calling him an immature child hiding behind a mask, a statement that left Kylo very upset. Despite the setbacks, Snoke becomes delighted to hear that Hux found a way to constantly track down the Resistance fleet and deduce their current location through hyperspace, so he orders both Kylo and Hux to lead a series of assaults on the Resistance fleet.
During the initial attack on the Resistance ship called the Raddus, Kylo uses his own TIE fighter to attack, but relents after learning that his mother Leia is onboard the ship. However, his wingman relentlessly continued the attack on the Raddus, killing everyone onboard, though Leia saved herself by using the Force to pull herself to safety.
Around the same time, Kylo and Rey have developed a certain connection to each other within the Force as they can communicate through their minds. After seeing the conflicted nature inside Kylo and learning about Luke's supposed 'betrayal' that caused him to turn to the Dark Side, Rey decides to bring Kylo back into the Light Side in hopes that the war would end peacefully. To that end, Rey surrenders herself to Kylo in the ship Supremacy, and he takes her into custody.
Being brought over to Snoke's throne room occupied by Snoke and his Elite Praetorian Guards, Rey is horrified to learn that Snoke plans to use his Force skills to torture her into giving out the location of Luke so that he can destroy the Jedi legacy and cement the First Order's reign across the galaxy. Snoke even confessed that he used the connection between Rey and Kylo as part of his plan. As Snoke tortures Rey, he sadistically orders Kylo to finish her off by gloating of his ability to read Kylo's mind and calling the latter a 'foolish child' for his hesitation. However, this finally drove an angry Kylo to rebel against Snoke by activating Luke's lightsaber and using it to cut Snoke in half by the waist, killing him instantly in an act of treachery. After helping Kylo finish off Snoke's guards to their deaths, Rey tries to convince him to return to the Light as he finally killed off Snoke for good. Unfortunately, Rey is horrified to learn that Kylo has no intention in returning to the Light as he wants to build a legacy far more different from that of either Snoke's or Luke's. Ren then demands that Rey join him in his pursuit, but Rey refuses and escapes, and Kylo frames her for the murder of Snoke to cement his role as the new Supreme Leader. He even Force-choked an arriving Hux into compliance when the latter refused to oblige to the new chain of command.
Directing the First Order's new attack to the planet Crait where a hidden Resistance base is located, Kylo orders his forces to attack the base. After the base is breached, Kylo decides to face an arriving Luke by himself. Engaging his uncle in combat, Kylo manages to slice through him, only to realize that Luke was actually projecting himself through the Force from Ahch-To as part of his true plan to let the remaining Resistance members escape. As Luke dies in peace, this left Kylo very upset that he failed to seize his chance of destroying the remaining Jedi, knowing that Rey will continue on with Luke's teachings to future generations.