According to the Instruction Booklet for Rocket Knight Adventures, Mifune was severely injured by Axel Gear, who had betrayed the Rocket Knights, during Axel's attempt to steal Ancient texts containing important secrets of the Rocket Knights. However, the book never made it clear if Axel killed Mifune. His fate is described from being "destroyed", "permanently disabled", and "severely injured" in different parts of the game's back story. Many assume that he eventually died from his injuries as he is never seen in the games, nor is there any official artwork of Mifune Sanjulo.
A Rocket Knight Leader, who at one point adopted and raised Sparkster to become a Rocket Knight after Sparkster became orphaned because of an unspecified war that took place. One can assume that this war had to do with the Devotindos Empire.