Even though Kylo Ren uses a red lightsaber, harnesses the dark side of the Force, and wears a cape, he is not a Sith. Because of that, he can never hold the title of Darth. Instead, he was just a chaotic mess of the dark side (until he eventually found his way back to the light).
He isn't a sith. He was never knighted and given the “darth” title. The dark lord of the sith (Palpatine disguised as Snoke) never initiated Kylo into the sith, all he did was partially turn him to the darkside.
Character. Abrams told Empire in August 2015, "Kylo Ren is not a Sith. He works under Supreme Leader Snoke, who is a powerful figure on the dark side of the Force." Abrams had previously stated that the character "came to the name Kylo Ren when he joined a group called the Knights of Ren."
Organization. Darth was a title given to certain Sith Lords which preceded either their chosen Sith name, or, in some cases, their birth name.
Having revealed that he used to be a Jedi Padawan under Luke Skywalker, Ben Solo changed his name to Kylo Ren in order to escape his lineage after turning to the dark side.
Because “Darth” a title that preceded the moniker of a Sith Lord. Kylo Ren was no Sith Lord, he was a Knight of Ren, an enclave of masked warriors who were neither Jedi nor Sith, but members of a new generation of Force-sensitive adepts.
Ben Solo was a human male Force-sensitive who fell to the dark side of the Force as Kylo Ren—the "Jedi Killer," master of the Knights of Ren, and eventually became the Supreme Leader of the First Order—but he returned to the light side shortly before his death.
As the first Dark Lord of the Sith, Ajunta Pall founded the first Sith Empire and expanded it onto other worlds. The Sith took over the planet Ziost and created it as their capital and as Pall's new home.
Sidious may have given Dooku the title of Darth Tyranus, but because the Jedi Order knew him as Count Dooku, greatly preferring to not indulge his role as a Sith, they continued to call him by that name.
Darth Rey, born Rey Palpatine, was a Force-sensitive human female who served the Sith Eternal during the New Republic Era.
You may have noticed that Snoke does not have the “Darth” title like the Dark Lords of the Sith do. He was never given one because he isn't technically a Sith Lord.
Darth Vectivus was one such Sith Lord. Vectivus was a businessman involved in mining operations, who just so happened to abide by a code of ethics. Though he was ruthless in business and had a vast wealth of Sith power, he maintained a level of balance and decency in his life.
Kylo Ren is strong in the Force, but he isn't the strongest there is or ever was. There are weaker folks, but many others could take him in a fight.
Darth Sidious was the last Sith Lord to exist in Star Wars canon, but does that mean the Sith threat is truly over in a galaxy far, far away? For thousands of years, the Sith laid waste across the Star Wars galaxy while facing off against the Jedi Order.
Emperor Vitiate was a Sith Lord who successfully gained near immortality by draining the life force of an entire planet, later attempting to drain all life in the galaxy, which would give him true immortality.
Darth Shain, headmaster of the Korriban Temple, took notice and convened with the Dark Councilor and Dark Lord of the Sith Darth Shane McMahon, who decided to confer the title of Sith Lord to the young apprentice at the age of 11, making him the youngest Sith Lord in the history of the Sith.
Jedi like Luke Skywalker and Obi-Wan Kenobi have long stood as light against the darkness. But the record Sith-killer is actually Darth Bane himself. Darth Bane, founder of the Rule of Two, killed more Sith than any Jedi in the history of the Star Wars galaxy.
Darth Nihilus is an iconic Sith Lord that many Star Wars fans adore. He is most known for the Sith Lord with one of the best “looks” since he wears a skeletal mask with a dark shroud.
Palpatine rules the galaxy for over two decades before Vader betrays and kills him to save his son, Luke Skywalker.
In Star Wars Legends and Canon, Galen Marek (Starkiller) is the only Jedi (other than Obi-Wan Kenobi) to put a beat down on Darth Vader/Anakin Skywalker. Galen was secretly trained as a Sith Lord apprentice to Vader, but returned to the light side and the Jedi.
“So Leia Force projects herself as Ben and uses her life force to save Rey. Even going as far as to kiss Rey to convince her it's Ben. It's why when Ben disappeared, so did Leia.”
Kylo Ren dies on Kef Bir (the ocean moon of Endor) shortly after Rey deals a mortal wound during a lightsaber duel on the wreckage of the second Death Star. Though Rey Force-heals him, saving his life physically, this blow, coupled with the memory of his mother and father, destroys the man known as “Kylo Ren” forever.
Darth Sidious was the Sith Master who trained several apprentices during his reign as the Dark Lord of the Sith. Approximately a millennium before the Invasion of Naboo, the Sith's greed for power led to infighting, which caused them to be defeated and nearly destroyed by the Jedi.