There are likely other characters from the First Order. For example, General Hux. His rivalry with Kylo Ren and his own ambitions within the First Order contribute to the overall plot. Also, characters from Kylo Ren's past, like his parents Han Solo and Leia Organa, are important even if they may not be as present in the novel as in other parts of the Star Wars saga, but their influence on him is still a big part of his story.
Darth Vader was once Anakin Skywalker, a Jedi with great potential. But he was lured to the dark side by Palpatine. He became a Sith Lord. Vader was a fearsome figure in a black suit with a red lightsaber. He carried out the Emperor's evil commands, like hunting down Jedi. However, in the end, he redeemed himself by saving his son, Luke Skywalker, from the Emperor's wrath at the cost of his own life.
Darth Vader was once Anakin Skywalker, a Jedi with great potential. He was lured to the dark side by Palpatine. His fall was tragic. He became a Sith Lord, a powerful enforcer for the Empire. He wore the iconic black suit and had a menacing presence. He was involved in many battles, like the destruction of Alderaan. In the end, he redeemed himself by saving his son, Luke Skywalker.
Kylo Ren's review of 'Solo: A Star Wars Story' would likely be very critical. Given his allegiance to the dark side and the nature of the story which focuses on a more light - hearted adventure of Han Solo, he might see it as a deviation from the true 'power - centric' Star Wars essence. He may not appreciate the origin story aspect as he is more interested in domination and the Sith ideology.
As of now, 'Kylo Ren A Star Wars Story' doesn't exist in a completed form. However, if it were to be made, it should start from his childhood. Ben Solo, who later became Kylo Ren, grew up in a family full of heroes. His story could show how the pressure of living up to their legacies, along with the influence of external forces like Snoke, led him astray. It might also detail his experiences in the First Order, his interactions with the Knights of Ren, and his ultimate quest for power within the dark side while still having a glimmer of the light side tugging at him.
He might be shocked by the advanced technology. The Empire's weaponry would seem primitive compared to future tech. He could also be intrigued by the new forms of the Force if they had evolved.
Well, it's likely that there are bounty hunters. Since Vader often has need of their skills for certain tasks. They would be characters who are in it for the money but also fear Vader. And then there are the stormtroopers. They are a constant presence, following Vader's commands and being part of his military operations.
One great 'Star Wars Darth Vader Fanfic' is 'Vader's Redemption'. It shows a different side of Vader, exploring his inner conflict in a very engaging way.
It's about Darth Vader's journey in the Star Wars universe. It might show his fall to the dark side, his relationship with the Sith, and his role in the Galactic Empire. It could also explore his inner conflicts.