On October 30, 2012, Disney acquired Lucasfilm and everything Star Wars related there was to acquire that came with it. This included all the of Expanded Universe, which Disney promptly kicked square in the super-expanded ass, booting it all into the trash can. It's now called Legends and I guess, acts as sort of an alternate universe to the canon of today. An alternate universe that will never be revisited with anything new, but we can always look back and revisit our favorite bits and pieces of it whenever we want (HELLO TRAITOR I AM HERE TO VISIT YOU AGAIN ARE YOU SICK OF ME YET) -- it'll just all be under the Legends banner now.
Now, I'll always be a little salty about all that, but I understand why they did it and am also a fan of the new movies. Finn is a perfect, precious bean and deserves the universe BUT THAT'S BESIDE THE POINT HERE~
When The Force Awakens was released on December 18, 2015, fans were introduced to a slew of new characters at the same time old favorites were being reintroduced. There were speculations of all sorts before the movie was released about character's relationships and all that, and the bomb was dropped (well, not exactly -- a number of fans expected this family relation) that Kylo Ren was Ben Solo, son of Han and Leia, and well on the path to the dark side.
BUT IT'S JUST LIKE JACEN SOLO!
Wait. I know where you're coming from because I said the same thing. I was a little bitter that the EU got shafted and we were experiencing sort of a Legacy-of-the-Force-Lite thing here. People completely disregarded that Jacen Solo was ever a thing and were all ~WELL HE DOESN'T MATTER ANYMORE CAUSE OMG KYLO SO HOT AND WOOBIE~ and I was frustrated because Han and Leia already had a son who did this who was tossed in the trash with his two also-extremely-A+ siblings.
The more time went on though, the more I paid attention to their differences and now I'm on the other end of the spectrum, when people say OH YEAH KYLO REN U MEAN JACEN SOLO????
I say, "No, I don't, because they're two totally different characters." Yes, they are both sons of Han and Leia and both went to the dark side. It stops there. I guess they both killed a family member as well. Although, Jacen/Caedus' was sacrifice related and Kylo's was...? And I guess Kylo's real name is Jacen's cousin's name? Since it seems like Luke naming a kid Ben is more understandable than Han and Leia naming their kid after the "crazy old dude from Tatooine."
We can't compare them both completely yet, as all we know of Kylo's journey is that Snoke is a creepy, manipulative dickhat who is corrupting the hell out of this kid and has been for some time. While Jacen, on the other hand, went to the dark side for selfless reasons -- a character trait he had possessed for many (probably most) of the books he appeared in.
Like the Yuuzhan Vong who had once held him captive, Jacen had become capable of any sacrifice -- and just as intolerant of those who did not share his commitment. Jacen Solo had fallen not because he was selfish, Luke realized, but because he was selfless. (Legacy of the Force: Tempest, pg. 236)
Jacen didn't want to be his grandfather and use his power to save one person, he wanted to use it to save his daughter, his lover and the whole damn galaxy. Sure, his actions were on the "what the shit is this kid thinking?" side, but his original reasons were frighteningly pure and that was one of the reasons I found him and his subsequent fall intriguing.
We've seen Kylo fanboying his grandfather and talking to his burnt helmet, so there's that, I guess.
Don't get me wrong, I am interested in Kylo and seeing where his story goes and where/how he got messed up, but as of right now Caedus' reasoning and storyline were more heart-wrenching. Yes, killing Han was heart-wrenching as shit too, but what was behind it? "lol snoke told me to do the thing~"
So yeah, Kylo Ren and Jacen Solo definitely have some similarities and I can see why people are always pulling the "BUT DO U MEAN JACEN SOLO" card, but the similarities are only on the surface right now and we have to wait to see where Kylo's story ends up before that card gets put into play again.
(Also, it might just be me but I feel like Caedus is a much more badass name than Kylo Ren, which people have easily turned into Crylo Ren, Kyle Ron, Kylie Renner, etc. And the public got to pick/vote on Caedus' name too!)
(Also, also: when fandom -- for fic/stories or RPs and whatnot -- completely take Jacen out of the equation and have the Solo Family be Han, Leia, Ben, Jaina & Anakin -- basically replacing him with Kylo. plz no.)