When Disney first began to tease the sequel trilogy that began with Star Wars: The Force Awakens and introduced a new wave of characters, fans of the Expanded Universe were quickly drawn to the dark Sith known as Kylo Ren, who would soon be revealed as Han Solo and Leia Organa's son, Ben Solo.
The Expanded Universe of novels, comics, and video games that Disney rebranded as Star Wars Legends continued the adventures of the original trilogy characters quite differently than the cinematic Skywalker Saga, but there were also a lot of similarities. The character of Kylo Ren especially seemed like he was pieced together using elements from one of Star Wars Legends' most powerful and tragic characters, Jacen Solo, or as he would come to be known when he joined the Sith, Darth Caedus.
Han Solo and Princess Leia were married and had Jacen and his twin sister Jaina, who were both destined to become very strong in the Force. Jacen and Jaina first appeared in Timothy Zahn's celebrated The Last Command novel, which was a part of the Heir to the Empire trilogy that also introduced the fan-favorite Grand Admiral Thrawn and was later adapted by Dark Horse Comics. Jacen and Jaina would go through many adventures together as they grew up and began to experience the Force alongside their younger brother and final addition to Han and Leia's EU family, Anakin Solo.
Unfortunately, a catastrophic invasion from a powerful alien race known as the Yuuzhan Vong would begin to chip away at Jacen's family, as both Chewbacca and Anakin were lost in the resulting galactic war. The Yuuzhan Vong came from outside the known galaxy and were unable to be sensed through the Force, which put Luke Skywalker's new generation of Jedi (that also included Jacen and Jaina) at a serious disadvantage.
After Anakin's death, Jacen was captured by the Yuuzhan Vong and tortured aboard their living starships in a device that repeatedly brought his body to the point of death while attempting to indoctrinate him into the Yuuzhan Vong's religion. While he was able to resist their alien teachings and eventually able to escape, it led to doubts about his place with the Jedi and their involvement in the galaxy's affairs.
Jacen was later empowered by the Force to destroy the leader of the Yuuzhan Vong, which ignited his belief that he was above notions of good and evil, and after he witnessed a future vision of a dark threat ruling the galaxy, he dedicated himself to saving the galaxy by any means necessary. This was further driven by the revelation that Jacen had fathered a child named Allana Solo with his fellow Jedi student Tenel Ka Djo. He kept his new family secret, which mirrored his grandfather Anakin's own ill-fated decision and further led Jacen towards the dark side.
Jacen vowed to learn everything he could about the Force in order to save the galaxy and his secret family from his dark visions, which led him to the Sith known as Lumiya, who trained him and set Jacen on the path to find a Sith apprentice of his own, which he tried to find in young Ben Skywalker (Luke and Mara Jade Skywalker's son). However, after Jacen's secret family and his suspected Sith turn was uncovered by Mara, he murdered her which turned young Ben against his former master.
This finalized his turn to the dark side and his evolution into the Sith Lord known as Darth Caedus, which he kept secret as he subtly gained more political power in the continuing galactic wars that eventually resulted in his takeover of the entire Galactic Alliance. This led to a tragic confrontation between Darth Caedus and his twin sister, now Jedi Knight Jaina Solo, that revealed a spark of light remained within him just before his death at his sister's hands.
When looking at the key points in Jacen Solo's rise and fall as Darth Caedus it's easy to see a number of similarities to Kylo Ren, especially when considering Rey as a potential counterpart to Jaina Solo. Ben Solo trained under Luke Skywalker as a Jedi before he lost his belief in his uncle and the Force, which brought him under the influence of a manipulative Sith lord.
Jacen was also separated from his family and his ultimate turn to the Sith was dependent on his murder of a loved one before he could fully assume his role as Darth Caedus, which Kylo Ren's later murder of Han Solo also facilitated before his takeover of the First Order. Luke Skywalker also entered into exile after the fall of Darth Caedus, though this was done more as a punitive action for his failure as Jacen's teacher. Even Kylo and Rey's final battles and reignited light mirrored Jacen and Jaina's final moments in some ways, though it lacked the emotional impact of the twin's history.
While Kylo Ren was his own character with alternate motivations and a very different familial relationship, the Jacen Solo stories clearly laid the foundation for Ben Solo's rise, fall and ultimate redemption.