After the victory over the Empire, politician Leia Organa and former Rebellion general Han Solo, a war hero, married and had a child, Ben Solo.
He was their only child because, a few years after his birth, Han and Leïa separated, for reasons similar to all couples, but mainly because, after the war, they had ended up gradually becoming strangers. Leïa Organa became very involved in the creation of the New Republic. General Solo tried to accompany his wife, but soon gave up, and after trying a thousand and one things, he returned to contraband, the only thing he really knew.
In the middle of this, Ben was abandoned. His father had left home, and his mother was far too busy to spend enough time with her son.
Also, when Ben was about ten years old, Leïa made the decision to send him to the Temple of Ahch-To, where his brother Luke Skywalker began to train the new generation of Jedi.
There, Ben Solo quickly became the most gifted of the apprentices. Alas, he found himself just as abandoned on Ahch-To as he had been before. His uncle tried to treat him with total neutrality towards the other students, he found no comfort in being near him.
The other apprentices, on the other hand, immediately tried to befriend young Ben. And by becoming the best student, he also became the leader of their little group. However, going into adolescence, Ben Solo began to come into conflict with his uncle. He became proud, unruly, and his behavior ended up dividing the students. Some moved away from him, others continued to follow him, slowly slipping into blind adoration of their leader.
As Master Skywalker came to think of expelling young Ben and his group, information spread throughout the galaxy. Leia Organa and Luke Skywalker were discovered to be the children of former Jedi Knight Anakin Skywalker, more tragically known as the Sith Lord Darth Vader.
Upon hearing the news, Ben Solo decided to leave the academy himself; with his band - his "knights" as he called them.
Quickly, a gang called the "Kylo and the Knights of Ren" began to make themselves known in the galaxy, by multiplying the thefts. The young Ben Solo, who has become Kylo Ren, ends up being caught during a burglary. He did not surrender without a fight, and two men lost their lives through his fault. Ben was convicted of murder and locked up.
Leïa, Han and Luke, despite the scandal that the fall of their son and nephew had weighed on them, felt no anger towards Ben. On the contrary, all three felt guilty for having abandoned him and being responsible for what he had become.
They visited Ben in prison. But the young man never forgave them.
He promised them that when he left, he would take revenge on them.
He promised them that one day he would be more powerful than all of them, as well as his grandfather.
Then the war broke out, and an opportunity eventually arose for Ben who took the chance to escape.
Outside, he learned of the death (or disappearance) of his uncle, and did everything to meet this famous Shadow.
Once facing Darth Nihilus, Ben Solo felt a power beyond his imagination. He who believed himself strong, capable even of exceeding the power of his family members, bowed to Nihilus, and begged him to take him on as an apprentice.
Thus, Kylo Ren - and his Knights, whom he brings together - became the right arm of the Shadow.