The Controller of Planet X (Ｘ星人統制官?, lit. X Alien Control Officer) is, as his title implies, the leader of the Ekkusu Seijin Tōsei-kanXiliens in Invasion of Astro-Monster and temporarily in Godzilla: Final Wars before he is murdered by his subordinate X during an interview with Anna Otonashi.
When astronauts Glenn and Fuji arrived on Planet X, the Controller brought them into a complex under the planet's surface, where he stated that he and the Xiliens required the Earthlings' assistance. The Controller told them that Planet X was being terrorized by King Ghidorah, called "Monster Zero" by the Xiliens, and that they wanted Godzilla and Rodan to be brought to Planet X to fight him off. The Controller promised a miracle drug capable of curing cancer in exchange for the Earthlings' help, and asked Glenn and Fuji to speak to Earth's leaders. The Controller secretly came to Earth in an Xilien UFO, hiding under Lake Myojin. Eventually, the Xiliens' presence was discovered by the humans and the Controller arranged a meeting with Earth's leaders. The Controller revealed the location of Godzilla and Rodan, and had two UFOs capture them. The Controller then invited Glenn, Fuji and their commander Dr. Sakurai on board his ship to witness the plan take place. When they arrived on Planet X, they witnessed Godzilla and Rodan battle and defeat King Ghidorah, with the Controller proclaiming it to be a great victory. He then sent the three back to Earth on a replica of their spaceship, the P-1. When they returned to Earth, they played the tape supposedly containing the cure for cancer, but instead discovered it contained an ultimatum from the Controller, demanding the Earth surrender to Planet X. The Xiliens soon besieged the Earth with their UFOs and the three mind-controlled monsters, Godzilla, Rodan and King Ghidorah. Using the A-Cycle Light Ray Gun, the people of Earth severed the Xiliens' control over the monsters, and then broadcast a sound across the Earth that was lethal to the Xiliens. The invasion a failure and his ship on the brink of destruction, the Controller proclaimed that the Xiliens would escape into the future and pressed a button, which caused all of the Xiliens' ships and bases on the Earth to explode.
The Controller revealed himself to the people of Earth after the Xiliens teleported away all of the monsters attacking Earth's cities. The Controller claimed that he had come to Earth to save the planet from a runaway asteroid called Gorath that would soon collide with the planet and destroy it. Working together with the United Nations Secretary General, the Controller replaced the United Nations with a new organization called the "Space Nations." All was not as it seemed though, as the Xiliens had actually come to Earth to harvest the humans' mitochondria in order to survive, and were secretly capturing Earth's leaders and replacing them with disguised members of their race. The Controller's protege, X, wanted to simply use their superior weaponry and the army of monsters at their disposal to bring humanity to its knees and enslave it. The Controller expressed a desire for peaceful methods, warning X that someone who let his life be ruled by violence would be destroyed by it. During an appearance on Anna Otonashi's live TV show alongside X and the Secretary General, the Controller was confronted by several members of the Earth Defense Force, who exposed the Xiliens' replacing of Earth leaders by shooting the Secretary General, causing his head to split open and revealing him as an Xilien impostor. The Controller tried to defuse the situation, but was shot and killed by X, who proclaimed himself as the new leader of the Xiliens.
In Invasion of Astro-Monster, the Controller is cold and manipulative, allowing his life to be run by a computer that controlled the lives of all Xiliens. The Controller was willing to conquer the Earth peacefully, giving the human race time to surrender without any bloodshed, but was more than willing to order the deaths of countless humans when they would not comply.
In Godzilla: Final Wars, the Controller is overall shown to be a good leader, putting the people's needs before his own. He could even be described as compassionate, as he wanted to take over the Earth in secrecy, with minimal destruction and death. It is possible that the Controller really did want to live in peace with the humans, and collect mitochondria in secret.