Lowbacca is Chewbacca's nephew. He went to the Jedi Academy on Yavin 4 and trained to become a Jedi. That was where he met the Solo twins and they all went on their crazy adventures. During the NJO, Lowbacca spent more time with Jaina than Jacen, so they didn't have much interaction. In the YJK, he spent a lot of time with Jaina also, tinkering with mechanical things. He and Jacen seemed to get along perfectly though when they were together. In a way, Lowie looks after the twins like Chewie always looked out for Han.

Zekk was another buddy from the Academy days. Jacen and his sister helped Zekk steer clear of the Dark Side, which he later learned to stay away from himself. The two became good friends and Jacen definitely talked to Zekk a lot later. Especially during the strike team mission to Myrkr and afterwards, when he spoke to Zekk after the knighting ceremony.

Tahiri is practically another Solo child now. Best friend and childhood love to Jacen's little brother, Tahiri and Jacen share a bond that relates to the fact that they both spent time with the Yuuzhan Vong. While Jacen can feel no pain, it's shown in The Joiner King that Tahiri is almost the same way, but sometimes she craves it. The two are good friends and even though Anakin's dying words were, "Take her Jacen. Kiss Tahiri for me." -- he didn't mean it like that and their relationship is nothing more than a close friendship spliced with the love that he shares with his family.