if you don't know anything about star wars then you wont understand this. Clone trooper Ghost was part of the alpha batch of clone troopers. those were some of the first clone troopers, and were the most advanced being taught by Jango Fett. They also didn't have the biochips. He disobeyed order 66 and saved his jedi master, and they went into hiding on mandalore. while there he developed a prototype serum to slow down his aging process. it worked but it also slowed it down past normal so he aged really slow. He then joined the rebel alliance and works for them as a special operative.
