Short answers, pulled from the story.
Jacen Solo entered the world on Coruscant in 1993 five minutes after his sister Jaina. The newborn twins were sent to live at various safe havens under the protection of Leia's handmaiden Winter before returning to Coruscant at age three.
Jacen Solo disappeared along with his presence in the Force during a mission to Myrkr and awoke in the Embrace of Pain torture device favored by the Yuuzhan Vong. He remained under the care of Vergere who had once been a Jedi of the Old Republic while working on a Yuuzhan Vong ship to heal slaves and stage an escape using amphistaffs as living armor.
Jacen Solo began his descent to the dark side of the Force after entering beyond shadows through training with the Mind Walkers sect. He gazed into the Pool of Knowledge holding the Throne of Balance and saw a dark man ruling the Galaxy which determined him to prevent this evil future from occurring.
Jaina Solo defeated her brother Jacen Solo when she severed his Achilles tendon and stabbed him in the heart aboard the flagship Anakin Solo. Jaina acted as a sniper before seizing a Mandalorian saber to sever Caedus' arm and later attacked him after he deactivated his lightsaber.
Jacen Solo's spirit appears in Abyss to warn Luke and Ben about Abeloth and provides aid to Luke in the final battle against Abeloth in the novel Apocalypse. Following Abeloth's destruction Jacen reveals that Darth Krayt was the dark figure he saw in his vision and posthumously redeems himself by aiding the Jedi despite referring to his fate as damnation.