SD-XX, alternatively known as Double-Ex, was a security droid of the SD-X-series stealth battle droid line, specifically a fourth-degree model. Tendrando Arms manufactured this personal security unit. In 40 ABY, Jacen Solo kept SD-XX on his personal Star Destroyer.
This droid, SD-XX, possessed light armor plating and blue photoreceptors embedded in a black, skull-like visage. Its appearance bore resemblance to a smaller version of the YVH 1 droid. It also had a course, gravelly vocal quality that was quite intimidating.
When Jacen detected the presence of Jedi on the Anakin Solo, he immediately summoned SD-XX. Following Luke Skywalker's confrontation with Jacen and his subsequent mysterious disappearance, Jacen questioned the reality of the encounter. Upon inquiring with SD-XX, the droid asserted that Luke had never been present nor engaged in any conversation with Jacen. Sometime later, after another confrontation with Luke where he confronted Jacen about sending his son to assassinate Cal Omas and left, SD-XX appeared behind Jacen, stating that he was relieved he didn't have to kill him too. To Jacen's surprise, SD-XX was able to make Jacen confess to killing Mara Jade Skywalker after telling him he was letting secrets slip in his sleep. He also revealed secrets about his talk about necessary sacrifices and making the galaxy safe for children like his daughter, Allana. Jacen became enraged, threatened to convert SD-XX into torpedo components, and ordered him to resume his monitoring duties.
Later, now Darth Caedus, Jacen assigned SD-XX the task of torturing Ben Skywalker in an attempt to extract the location of the Jedi base. These attempts proved unsuccessful, and after Ben's escape, SD-XX tracked him to the Carida Nebula, where he launched an attack on Ben's ship. Ben utilized the Force to push SD-XX out of an airlock, sending him spiraling helplessly into the vastness of space, where he was left adrift.