Lassin was a member of the New Jedi Order and a Jedi Knight, who met his death not long after the Disciples of Ragnos situation, circa 14 ABY. In 41 ABY, a Force vision occurred in which Lassin, together with Mara Jade Skywalker, Jedi Master Kam Solusar, and the Sith Lady Lumiya, appeared to the Jedi Knight Jaden Korr. During this vision, Lassin and the others displayed Force signatures that resembled those of the Sith more than the Jedi, and they instructed Korr to travel to the planet Fhost. As a result, Korr uncovered an Imperial research center located on a frozen moon, where Grand Admiral Thrawn had instructed scientists to create clones by fusing Jedi and Sith DNA. Inside this facility, Korr came across a clone of Kam Solusar, leading him to suspect that the other clones might have originated from the DNA of Lassin and the other figures who appeared in his vision.
The first appearance of Lassin was as a vision in the novel Crosscurrent, which was written by Paul S. Kemp and saw its release in 2010.