Before the events of FusionFall, Samurai Jack took on a pupil named Yago. Yago was extremely strong; however, he had a flaw that Jack did not know about. Yago had already had a master and a dark past, which his old master discovered and confronted Yago about. His pupil, however, had become too strong ― his old master could not defeat him. Yago fought him and thought he had killed him, but had simply given him amnesia.
As it happens, Yago's old master was none other than Jack himself. Jack has gotten stronger, and Yago knew that one day he would regain his memory. When Jack seemed to be recalling some details of their shared past, Yago grew frightened, and decided to bolt.
A New Master
Having left Jack, Yago wanted a new master, so when he sighted Planet Fuse, he immediately decided to escape to there, and went to the one place he knew of where he could get a space ship: DexLabs! He sneaked in to through a ventilation shaft, but heard Computress, Dexter, and Ben talking about Planet Fuse. Deciding that Lord Fuse would be the perfect master, he stole a space ship and headed for the planet.
It is unknown if Yago became a minion of Fuse's. There have been no recent sightings of him and so it is thought by some that he is dead.