Here's the section on Jacen's love interests. One of them probably shouldn't be considered a love interest. But I put her in here since she was a close friend and very important to Jacen?? Heheheehhehe.
Tenel Ka Djo was Jacen's long-time pal. They met at Uncle Luke's Jedi Academy where their friendship grew over time. In a nasty lightsaber training accident (TK's fault due to her rushed/faulty lightsaber build), Jacen sliced off Tenel Ka's arm. He's always held that guilt inside of him and he always blamed himself for it -- even though she always told him it was her fault.
The cute little semi-couple in the Young Jedi Knights series and a who-the-hell-knows in the New Jedi Order (thanks to dueling author opinions on his love life, I guess?), left people wondering just where Tenel Ka would end up in Jacen's life at the end of the NJO and beyond. Parts in the series showed that Tenel Ka cared for Jacen more than even Jaina could imagine, but did Jacen care for her the same way?
Dark Nest: The Joiner King came along and gave us a meaty chapter full of Jacen and Tenel Ka. He goes to visit her to ask for a fleet/help with the galactic problem at hand and he ends up staying the night. There were people saying that they were using each other (him to get the fleet, her to make her suitors jealous) -- but others who had read the YJK and know the background of these two say that "they always banter like that" and "they would never use each other like that, especially because of the way they feel about each other" ... so there was that debate. There was also the debate of who was going to have the rumored baby in the Dark Nest trilogy. A lot of signs pointed to Miss Tenel Ka -- after the fact that Jacen spent the night with her and Han's mention of how Jacen was "good with kids"...
Both of those debates were put to rest once the Dark Nest Trilogy was fully released. In the second book, Tenel Ka and Jacen flat out tell Ben they are in love and they always have been and now they're parents! Tenel Ka had a baby and right after Jacen was introduced to the kid -- who is only a week old at this point -- (and who didn't have a name until the next series), the new parents had to fight off an attempt on her life (thanks Ta'a Chume). Now, into Legacy of the Force territory, the child is still alive and kicking, her name is Allana (named after both Tenel Ka's grandmother, Allaya, and Jacen's late brother, Anakin) and she has her mommy's looks. Awww, Jacen has a little spawn. He was traveling on a pretty fine line for a good while, with Lumiya telling him he needed to sacrifice what he loves most to become a full Sith and save the galaxy.
He takes his new name after killing Mara Jade Skywalker and later, once Tenel Ka finally takes Hapes off of the Galactic Alliance's side, Jacen -- now Caedus -- realizes that Allana is the only person left who still loves him. And he would kill anyone who gets in the way of them, even Tenel Ka.
By the end of the series, once he's too far removed from his smarts, Caedus and his twin sister, Jaina, have their last set of battles. No one bothers to try and redeem him and as his sister is in the middle of fatally stabbing him, he calls out through the Force to Tenel Ka to warn her of the approaching dangers she and Allana will be facing. He basically saves them both from an attempt on their lives. He had a choice -- he could've saved the two of them in that moment or taken the brief moment to fatally wound his sister and he chose Tenel Ka and Allana. In the end, Jacen got his last har har and called out to Tenel Ka, cause you know, he cared about family, unlike his sister and his parents. Ohhhhhhhh, still bitter about LotF, party of one. (If you want my true feelings about Tenel Ka, you can read this powerpoint presentation I made about her.)
Danni Quee was Jacen's older woman interest. She was a scientist for the New Republic. Jacen saved her from Yuuzhan Vong captivity very early in the NJO and he had a small crush on her afterwards. She said they should just be friends and he accepted that quietly, the relationship falling off of the radar until much later in the series. (I guess, when he was legal everything changed and she suddenly wanted more? I mean, that's what the timing made it look like.)
She was absent from Jacen's presence until New Jedi Order: Destiny's Way, when the two went diving together during Jacen's vacation. The scenes looked strictly friend-related, but it seemed to be turning into more during the Force Heretic trilogy. Throughout the FH series, Danni kept letting Jacen know how she felt about him, and Jacen kept acting oblivious towards it. Because Jacen.
By the end of the NJO, Jacen seemed to shrug off mention of her casually, claiming to Jaina that 'he knows where she'll be' -- and Danni realized she didn't love him, the one line from her perspective closing the book on any romantic relationship:
"She owed her life to Jacen, and at one point had come close to falling in love with him."
Vergere was Jacen's mentor/teacher and also could be considered a student/friend. She met Jacen in Star by Star and from there they had a very rocky relationship.
Her constant riddles and crazy talk left Jacen utterly helpless in Traitor, until he realized that her crazy sort of made sense and then he owned everyone and a lot of Vong... but that's another story.
By the end of Traitor she proved to be a useful ally and an even better friend. After her death in Destiny's Way, Vergere talked to Jacen like Obi-Wan talked to Luke. And she still played an important role in Jacen's life and shaping later when he started down the Sithly path with Lumiya.