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 booklet 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 backstory. 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.
He and King Zephyrus were described as very close friends that were going on many adventures during their youth.
- According to the Overseas Manual Mifune Sanjulo got "severely injured" by Axel Gear during his attempt to steal the "Secret Book of Rocket Knights".
- In the Japanese Manual, however, Mifune Sanjulo got killed by Axel Gear who successfully stole the "Secret Book of Rocket Knights". This incident ultimately caused the rivalry between Sparkster and Axel Gear.