Kylo Ren is a major antagonist in the Star Wars sequel trilogy. His real name is Ben Solo, son of Han Solo and Leia Organa, nephew of Luke Skywalker and grandson of Darth Vader. He is also somehow the leader of the Knights of Ren.
He's a ripoff of Jacen Solo that was created to jab at Darth Vader fanboys.
He is just an whiny, edgy, brooding petulant child that only shows surface-level conflict.
He is inconsistently portrayed. He is so powerful and disciplined he can stop the movement of a blaster bolt in mid-air for some time without any apparent effort and could defeat Luke Skywalker in a duel, while on the other hand he is shown as unstable, undisciplined and able to be overpowered by Rey, a girl who was unaware that the Jedi and the Force were real until earlier that day.
He is prone to tantrums that make him way less threatening than any other villain. The only villain who is a bigger joke than him is Hux.
Him being named Ben did not make sense as Han and Leia not as close to Obi-Wan as Luke was.
The removal of his mask in was also premature and underwhelming. And Ben Solo looks goofy without it. Its made worse in that he actually destroys his mask in Last Jedi. It is also the moment he lost his intimidation and scariness making him a grumpy emo kid.
His relationship with Rey and the whole "there's still good in him" felt forced.
There is no sound reason why he became a villain. Sure, he saw Luke thinking of killing him or even Emperor Palpatine manipulating him into leading the First Order, it doesn't explain why he turned on anyone else.
Adam Driver does do a good job with the material he's given.
His stopping a blast bolt is mid air is pretty awesome and shows how powerful he is.
He at least got his trademark mask back during The Rise of Skywalker.
His conflicted persona is quite understandable, as his outward appearance suggests he's just trying to play the part of Vader, but inside he's his father, his mother, and his uncle, making up a balance of Light and Dark.