The evidence isn't entirely clear, but it is most likely that he was an adherent of Form V, particularly the Djem So variant. Ren's fierce fighting style has the hallmarks of Djem So, which demands a high degree of speed and strength to execute a flurry of attacks and counterattacks.
Kylo Ren's lightsaber form was something of a hybrid, which blended three styles together: Djem So, Juyo, and Niman.
While Kenobi displayed a strong preference for Soresu, he continued to apply elements of the relentlessly dogged Form I as well as the erratic strokes and acrobatics of Ataru. At one point just before the conclusion of the Clone Wars, Mace Windu acknowledged Obi-Wan Kenobi as "the master of Soresu."
For much of the Clone Wars, Anakin used Form V in battle. Darth Vader also used lightsaber Form V, and the idea behind this technique was to end a fight before it could be allowed to begin. It was a heavy-handed style and executed with aggressive, powerful strikes.
One of the main reasons Kylo Ren lost so many duels in the Star Wars sequel trilogy was due to his lightsaber form. Lightsabers were the traditional weapon of the Jedi and the Sith with both sides adopting different styles to match their preferred way of combat.
After a career of pure darkness, weaknesses like sentiment were his undoing and the undoing of the Empire itself. Kylo Ren, while worshiping Vader, was afraid of replicating that weakness. As time went on he saw more and more of said weakness Kylo himself began to even find resentment in Vader, his hero.
The lightsaber's redesign was a byproduct of the bleeding process that damaged its kyber crystal, rendering it dangerously flawed and unstable, and necessitating the lateral vents were meant to prevent the cracked kyber crystal from overloading.
Lightsaber combat forms and sparring techniques are heavily inspired by kendo, a Japanese martial art that revolves around sword fighting. For example, Obi-Wan Kenobi's famed lightsaber duel with Darth Vader on the Death Star shows the influence of this fighting style.
The sole Form VII variant to have gained recognition by the Jedi Council, Vaapad, was only created in the final decades of the Jedi Order. The key architect of Vaapad was Jedi Master Mace Windu, who developed the form to address his weakness by controlling his inner darkness and channeling it into worthy ends.
To the Jedi of the Clone Wars era, the most prominent practitioner of Form V was Anakin Skywalker, who mastered both Form V variants. During the waning years of the Old Republic, Jedi Padawans often spent a year or two studying each Form from II to V.
Juyo. Darth Maul utilizing Juyo against Obi-Wan Kenobi's usage of Ataru. Juyo was an established form of combat four millennia prior to the Battle of Yavin. Jedi Masters Vrook Lamar, Zez-Kai Ell, and Kavar were all skilled in its use and were able to pass on their experience to others.
Yoda employs Form IV, which is called Ataru. As seen in his battles throughout the prequels, this particular form relies heavily on aggressive attacks and acrobatics, as well as an emphasis on using the Force to enhance their movements, strikes, and agility.
Some lightsaber duelists used Teräs Käsi techniques in combat, most notably Darth Maul, who trained extensively in the art and incorporated it into his lightsaber and unarmed techniques - Maul credited the martial art as one of the primary reasons he was so successful and deadly as an assassin.
Kenobi is known to be a great master of Form III. He used this form to beat Vader in Mustafar. However, I believe Obi-Wan was also brought up with Form IV and he used this to beat Maul on Naboo. He only switched styles after witnessing Qui-Gon's death.
By the time of The Rise of Skywalker, Kylo Ren has grown enough to stay calm and show off some of his more technical fighting skills. Once he does, he shows that he's capable of being an extremely powerful lightsaber duelist...even if he loses that round, too.
Yes, Kylo Ren is a powerhouse, with undeniable abilities never before seen in the Star Wars universe; he personifies the power of the dark side. Fear is the ally of dark side wielders, as it increases a dark side user's strength through instigating fear.
There are 7 forms of lightsaber combat, but Soreso, Shein, and Vaapad are the most effective. Soreso, Vaapad, Shein, are the most effective forms because they are all strategic and have their strengths and weaknesses. But the thing they all have in is that you must fully commit to the form.
Form VIII: Kendosu, or the Way of the Varactyl, was a new swordplay style developed during the Great Galactic War by Jedi Knight Scott Borek.
Padawan Windu was trained in the ways of the Force by Jedi Master Cyslin Myr. Following his early trials as a youngling, Windu attained the status of Padawan by becoming Jedi Master Cyslin Myr's apprentice.
Qui-Gon Jinn and Obi-Wan Kenobi utilize Ataru against Darth Maul's Juyo on Naboo. Because it was such an aggressive style, Ataru was not generally optimal for use against multiple opponents, and it was somewhat ineffective against blaster fire, particularly from multiple foes.
Form 1: Shii-Cho
The basic "form" of lightsaber combat, and described in Legends material as a method focused on letting the mystical force guide a Jedi's movements, has been called "Shii-Cho." Most Jedi characters in the old continuity began their training with this form before moving to something more specialized.
Ahsoka Tano was also skilled in the use of Jar'Kai and wielded both a lightsaber and a shoto. She employed an extremely complicated Ataru-Shien reverse grip style to her Jar'Kai tactic that was worthy of mastery.
We learned in The Force Awakens: The Visual Dictionary that the now-infamous red lightsaber had a cracked kyber crystal. Thanks to The Rise of Kylo Ren #4, we now know that the broken crystal is a result of the bleeding process.
Kylo Ren's reforged helmet was strengthened through Sith alchemy. Sarrassian iron was painstakingly applied between the shards of the shattered helmet, creating a bright red alchemical web that resembled crimson lightning or scars.
A crystal embodies the Jedi as the heart of the lightsaber, and pure kyber has no color until it's bonded to the Force user. This process likely resulted in Rey Skywalker's lightsaber being yellow.