After watching The Last Jedi (2017), we can see the development that has taken place in the relationship between Rey and Kylo Ren. In the first part of the new Star Wars trilogy, The Force Awakens (2015), these characters face each other as enemies. Rey had something that Kylo Ren needed, a part of the map that showed the current location of Luke Skywalker, who had been hiding for many years and represented the last hope for the Resistance against the First Order.
Kylo Ren takes Rey prisoner and uses the Force to extract information from her mind. However, something unexpected happens that Kylo had never experienced before: he finds a blockage in Rey's mind, she resists and "pushes" him, and uses the Force against him to read his mind. This leaves him bewildered, intrigued... He didn't hate the girl even though she was on the side of his enemies, it simply caused him great interest. All this is best appreciated in the novelization of The Force Awakens, written by Alan Dean Foster.
After that first encounter, Kylo and Rey fight in the woods of Starkiller Base after Ren killed his father, Han Solo. His unbalanced and broken spirit (as Snoke would describe it to us in Episode VIII), together with the wound that Chewbacca caused him with his blaster, would lead to the defeat of the young Solo in front of an enraged Rey, who was beginning to discover her great ability and the Force within her. "You need a teacher!" Kylo tells her before he is defeated.
Why do I say in the title of this post that these two characters, so contrary to each other, are the balance in the Force? For this very reason, and in The Last Jedi we can see the signs of it.
Rey and Ben are the opposite sides of the same coin. They both possess the same power in the Force and this attracts them as it confronts them: "Darkness rises... and light to meet it," says Snoke. The power within Rey emerges as an equivalent to Ben Solo. This is a theme that is strongly emphasized in The Last Jedi and Luke explains it on the island while training Rey "powerful light, powerful darkness"; just as there is life, there is death, just as there is peace, there is violence. The Jedi have been reneging on the dark side for many, many years and this has led to their failure, when the real balance lies in finding the equilibrium between the dark and the light.
This is what Rey and Ben could achieve.
In The Last Jedi, we start with an important symbolism: Kylo Ren gets rid of his mask. In Episode VII, Rey calls Kylo "a creature in a mask," and he takes it off to reveal his true appearance. Perhaps this is the first time that Kylo Ren wants to be seen as a man, a human, and not a monster. By destroying his mask in The Last Jedi, Kylo Ren lets out his true identity, Ben Solo.
This is reinforced by Rey, who begins to call him by his real name when the two begin to connect through the Force. This connection, supposedly created by Snoke, allows the two of them to communicate no matter how far apart they are, and not only that, but they can both feel each other's presence, be affected by their surroundings (if there is rain, for example) and touch each other. And when their hands meet, they can see each other's future, a future where they join the same side and fight together, "where they would never be alone again," describes Rey in the novelization of The Last Jedi.
Upon discovering the true story of Ben Solo, who was betrayed by his uncle Luke Skywalker in an impulse from him to kill his nephew at the sight of a terrible vision where Ben was dominated by the dark side, Rey begins to empathize with the young man and firmly believes that he can be redeemed. Rey knows that the conflict destroys Ben inside and wants to help him; she calls him "our last hope."
Both are people who have been abandoned, in one way or another. Rey was abandoned by her parents as a little girl, and Ben was sent to train with his uncle Luke; in an attempt to keep him away from the dark side, Leia and Han achieved the opposite effect, distancing their son from them and giving Snoke a better chance to take advantage of this... Luke's betrayal was the trigger for Ben to join the dark side. As we can see, he didn't make the decision out of evil or the need to join the darkness. They are both adrift and have no one to understand them until they begin to connect and see themselves reflected in each other.
"You're not alone," Ben assures Rey. "Neither are you," she answers.
However, their paths would not be destined to unite (at least at this time). In what is probably the best scene in the film, Ben kills Snoke and fights alongside Rey to defeat the Praetorian Guard. Both show mutual trust and, by combining their skills, manage to beat the guards. Rey asks Ben to stop the attack on the Resistance; however, he had other plans.
Free from Snoke's oppression, Ben decides to take the First Order into his own hands and offers Rey to "rule the galaxy together." This breaks Rey's heart and in tears she asks Ben "not to go this way" (yes, this is a parallelism of the scene between Anakin and Padmé in Episode III, where Anakin makes a similar proposal and his wife answers "you are going down a path I can't follow.")
Rey and Ben face off one last time, they both try to take control of Anakin's lightsaber and it ends up breaking in half. In the last scene of the film, Rey enters the Millennium Falcon and connects with Ben once again through the Force, they both look at each other (not with hatred, but with pain, disappointment) and she "closes the door" between them. The last image we see of Ben Solo, the new Supreme Leader, is not that of an evil leader who has conquered the throne, but that of a man on his knees who is confused, lost (regretful even?).
Only Episode IX will reveal whether this is a love story destined to unite these characters, capable of achieving balance in the Force, or whether we will have a tragic ending. Until then, we can only speculate, what we do know is that the relationship between the two is getting closer and closer, and this new obstacle is nothing more than an impulse for them to meet (or confront) again.