Like the other Mysterians, the Mysterian Leader wears a full bodysuit and a helmet that covers most of his face. However, the Mysterian Leader's outfit is colored red to denote his rank, while his subordinates all wear either yellow or blue.
The head of the Mysterian's Earth invasion force, the Mysterian leader operated from inside the Mysterian Dome near Mount Fuji. When the JSDF uncovered the dome's location, the Mysterian Leader broadcast a message asking for a group of humans to enter the dome and meet with him. When the humans entered the dome, the Mysterian Leader had them take a seat and revealed his race's demands to humanity. He assured the humans that they sent their robot Moguera to destroy nearby villages only as a show of power to prevent the humans from trying to attack them in the future, and that his race were pacifists. He requested that the Mysterians be granted a two-mile tract of land on which to live on Earth, as well as the right to mate with Earth women so they could rebuild their population, as 80% of their population was born abnormal due to the presence of Strontium-90 in their bodies. He warned that his race possessed weapons far beyond humanity's capabilities, and that if the humans provoked a war with the Mysterians, it would end in humanity's destruction.
When the JSDF ignored the Mysterian Leader's requests and assaulted the dome, he directed the counterattack, which successfully wiped out the human forces. The Mysterian Leader ordered his subordinates to continue work on the dome, as upon completion it would allow them to easily crush any resistance from humanity. The nations of the Earth joined together to form the Earth Defense Force and attacked the dome again, but were again repelled by the Mysterians' resistance. The Mysterian Leader responded to the humans' continued resistance by broadcasting a message to mankind stating that the Mysterians now demanded a 75-mile plot of land, and warned of further retaliation if the humans continued fighting them. As the dome neared completion, the EDF launched a final assault against the dome with its advanced new Markalite Cannons, which could reflect the dome's heat rays back at it. When the Mysterian Leader learned that Joji Atsumi had breached the dome to rescue the women the Mysterians had captured, he ordered his men to stop them. One yellow-suited Mysterian led Atsumi and the women to the exit to the dome at gunpoint, only to reveal himself as Ryoichi Shiraishi, an astrophysicist that had been taken in by the Mysterians. Shiraishi had overheard from the Mysterian Leader that the dome would be able to level Tokyo when it was completed, so he decided to turn against the aliens and stop them. Shiraishi freed the captives and returned to the dome, damaging it from the inside and causing it to begin collapsing. With their dome about to be destroyed, the Mysterian Leader ordered for all surviving forces to retreat to the Mysterian Space Station. Shiraishi prepared to shoot and kill the Mysterian Leader and his top lieutenants, but decided against it and let them flee as the dome finally exploded. The Mysterian Leader presumably escaped to the Space Station in a UFO, doomed to live forever in the void of space with his race for the rest of eternity.
Yoshio Tsuchiya had an active interest in UFOs and space travel, and rejected the lead role of Joji Atsumi to play the Mysterian Leader, which Toho saw as a bit part due to the character's mask. It was his idea for the Mysterian Leader to speak in a robotic fashion, in sync with the low warble of his universal translator. Years later, director Ishiro Honda would note that the result was identical to the sound of the first real computer-generated voices.
- The Mysterian Leader is seemingly the only Mysterian capable of speech, as the other Mysterians never talk and appear to communicate exclusively through hand motions and body language. The Mysterian Leader still needs to use a universal translator to speak.
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