Rigel was originally the younger brother of Noulan, the hero who defeated Kaiser Hades. After Maia, the princess of his planet, was killed by the Rejander Drake, he had his memory wiped by Hades before being recruited into the Hades Army, where he was trained by the Death Commander Danhauser.
With Hades Army
Assuming the human alias of Shiro Jinno, Demon Knight executed a plan to fool the Justirisers into thinking that he thinking he was an ally. He lured Mio into a trap in order to find out more about the JustiPower so he could convey the information to General Bachuss.
After being exposed as a member of the Hades Army, Jinno became an enemy of the Justirisers. He briefly lost his ability to change into Demon Knight after he was defeated by Glen when the Justiriser achieved his full potential, but he regained it after Danhauser was fatally injured.
Jinno later learns the truth about himself after the appearance of Mira, prompting him to join the Justirisers as an ally against the Daruga Army following the death of Kaiser Hades. He would begin to develop feelings for Reika, Mio's bodyguard.
After Majin Daruga arrives to conquer Earth, Jinno winds up having his Riser Power stolen by Daruga, who uses it to become Dark Demon Kurogane. He would later discover that Drake was the one who killed Maia and killed him to avenge her.
During the final battle, Jinno adds his Riser Power to Mio's Justi power to give the Justirisers the energy to transform into Giant Shirogane to finish off Giant Kurogane. With Kurogane, the Hades Army and the Daruga Army defeated, Jinno bids farewell to Shouta, Yuka, Shinya, Mio and Reika as he pilots Ryuto to return to his home planet.