Like with Rey, the film takes a deep dive into the divide that exists within Kylo Ren, as he still struggles with the Light within him even after killing Han Solo. It's a conflict that's even more prominent in this movie, and terrifically explored through his interactions with Rey.
Adam Driver has carved out a reputation as a great actor, and his talents are on full display here. Through his trembling face and shaken voice we get to understand all of the pain, turmoil, and anger he's feeling, while also seeing displays of his considerable power too.
It's an intense, commanding performance from Driver, and one that constantly keeps you guessing what where Kylo will go and what he'll do next. He's not a hero, but it seems unfair to call him a villain; he instead becomes one of the most interesting and layered characters in the whole saga.
Warning: the next entry contains major SPOILERS!