Dr. Julien is an inventor and built Zane and the Falcon. He spent most of his life with his "son" and he taught Zane to do different things. When he was on his death bed, he told Zane he was made to protect those who cannot protect themselves and that he needed to move on to the next stage in life. Before he passed away, he switched off Zane's memory switch so he could forget his past. He also had a robot to protect their home from Treehorns, which is later destroyed by Zane, before he unlocks his full potential.
In the next season, it is revealed that he is still alive and held captive in a deserted lighthouse in the middle of the ocean. It turns out that shortly after his death, Samukai visited him and revived him with a potion, forcing him to build powerful machines that would aid him in attacking Ninjago. To avoid him escaping, Samukai chained a large sea creature at the bottom of the sea as a guard. He was found by the ninjas and was rescued in The Last Voyage.