Although it pretty much divided the fandom down the middle and gave rise to a pocket of unbelievable toxicity, the Star Wars sequel trilogy gave the lore some fantastic elements, including characters. While people like Finn, Poe, Rey, Zorii, etc. will continue to shine in future content, the standout of the three movies was consistently Kylo Ren.
Another tragic character, who had three of the most exceptional performances in Star Wars history, and a somewhat divisive but overall well-done redemption arc, Kylo was a great villain. As a villain, though, elements of him got worse and worse.
His Physical Scars
In The Force Awakens, during his lightsaber fight with Rey, Kylo gets a sizeable ugly scar, that later got changed to a cooler, less ugly scar down his face and neck.
Alongside that, though, Kylo has the scar from Chewie's bowcaster, visible for a slight second in the infamous Kylo topless scene. In terms of physical appearance, Kylo also looks far more stressed and drained throughout The Last Jedi.
His Resentment
To say that Kylo Ren has some parental issues is an understatement. He hates his father an unbelievable amount for two and a half movies, as well as his uncle. He also acts as if he hates his mother, but can never bring himself to do anything about it.
Throughout the trilogy, Kylo grows in resentment towards his family, towards Snoke, towards anything of the past, and even, at points, towards the only thing that seems to make him some semblance of good, Rey.
His Increasing Obsession
Resentment, though, is not Kylo's only longstanding emotional issue, as he demonstrates on numerous occasions, through various means, that he is obsessed.
He is not only obsessed with destroying everything he resents, as mentioned above but also obsessed with Vader, becoming Vader, finishing the iconic Dark Lord of the Sith's 'work.' Also, as many fans have pointed out, he appears, at times, to be obsessed with Rey.
His Use Of Violence
Kylo's violent streak can be seen from the literal opening of The Force Awakens, all the way to his final battle with Rey on Kef-Bir, and it has peaks and troughs of getting worse and worse and staying dormant for a little while.
He not only tortures Rey and Poe - and probably more people before that - but he issues an order to slaughter an innocent village, strikes down Lor San Tekka, massacres the Alzamec on Mustafar, and his bombardment on Luke's Force projection, as well as killing Han Solo, and numerous lightsaber battles.
His Total Instability
Something that gets increasingly worse throughout the first two movies, while staying at a consistent lesser yet evident level throughout The Rise Of Skywalker is Kylo's immense instability.
He has gotten compared to an emo teen with a lot of family issues, and his instability shown through things like his tantrums, destroying whatever he is next too with his lightsaber. Moments like the destruction of his helmet, the orders to fire on Luke, and then the falcon, the snapping at Rey for her continuing to hold on all point towards Kylo's unstable nature.
His Built-Up Emotion
This instability that comes in various bursts, as well as his resentment and obsession, has a lot to do with Kylo's built up emotion, and inability to deal with it in a safe and stable way. Something with which his grandfather also struggled.
The only time Kylo ever seems to show glimpses of the good within him is when he shares a scene with Rey. Kylo is so angry, and so filled with hate towards people who do truly love him, something that remains until the moment where the one family member he could not help but love, his mother, calls out to him and passes away.
Increasing Hunger For Power, Fear Of Any Threat To Losing It
Like most Sith, Dark Side Force users, and Star Wars villains in general, Kylo becomes a very power-hungry individual, and by the time of The Rise Of Skywalker, is clearly fearful about losing that power.
Little moments with Hux in The Force Awakens show that both are looking to be the higher ranked, and looked at more fondly by Snoke. The Last Jedi proves Kylo wants to be rid of the Resistance and Skywalker and any other threat to his power. Finally, in The Rise Of Skywalker, Kylo goes alone to face Palpatine to try and kill him, all because he views Palpatine as a threat to his power, only ceasing when Palpatine promises him more.
His Ranks In The First Order
That hunger for power Kylo has gets somewhat fulfilled in The Last Jedi when he kills Snoke and takes up the mantle of Supreme Leader for himself.
Kylo's rise in the ranks, not only as an apprentice of the Dark Side of the Force but in the tyrannical First Order, seemed to solidify the deterioration of the light within him, with The Rise Of Skywalker taking away from that idea.
Used By Palpatine
One thing that got revealed about Kylo in The Rise Of Skywalker was that he had merely been a pawn in the plan of Palpatine, who had been puppeteering Kylo's master, Snoke as a test for the young Skywalker.
The fact that Kylo had been getting used by Palpatine this entire time adds to the tragedy of his arc massively. But, from a qualitative standpoint, it hurt Kylo as a character having Palpatine's nonsensical return, not as bad as it hurt the film or trilogy, but still hurt.
How Torn He Is
One of the most common character themes in Star Wars is the internal struggle, being torn between two things. Han between his selfishness and the good within him, Leia between her emotion and her commitment to the cause, Luke between the wishes of his family and the wishes of himself, Rey between her desire to figure out the past and her desire to help the future.
Kylo exemplifies this torn temperament more than perhaps any other character outside of Anakin. He struggles with just about everything internally as the Dark Side and Light Side have a war within him. The final meeting with Han in The Rise Of Skywalker, which resembles exactly that from The Force Awakens, sees Kylo finally let go of that hate and darkness, something he struggled to do back then.