"X" is a name that he simply chose for himself, as he felt his real name would be too difficult for humans to pronounce. He felt the name was fitting because it captured his personality, as he considered himself mysterious.
X appears to be mentally unstable, as he feels no remorse in killing his superior, the Controller of Planet X, who is presumably his mentor. X is prone to untimely fits of hysterical laughter, like when he laments on the enslavement of the entire human race or when gaining the upper hand in a fight. X is also prone to violent outbursts of rage. He becomes very angry when one of his monsters is defeated, or when he is outsmarted by one of the humans, often to the point of him jumping up and down and flailing his arms in anger. However, these outbursts are unusually short, and he can snap between near-manic anger to a cool and emotionless state in the space of a few seconds.
X revealed himself to the people of Earth alongside his superior, the Controller of Planet X, claiming that he and the Xiliens had come to Earth to save the planet from an incoming asteroid called Gorath, that would soon impact and destroy the Earth. X instantly became a celebrity on Earth, making numerous appearances on television and radio shows. All the while, X grew impatient with the Controller, who wanted to harvest the humans' mitochondria through peaceful means with minimal violence, while X preferred to annihilate their civilization immediately using the Xiliens' superior technology and enslave them. Eventually, members of the Earth Defense Force discovered that the Xiliens were replacing high-ranking Earth officials with disguised members of their race. A group of them confronted X and his superior while they and the United Nations Secretary General were being interviewed live by Anna Otonashi. Douglas Gordon shot the Secretary General, causing his head to split open and revealing him as an Xilien. The Controller attempted to defuse the situation, but X simply shot and killed him. X declared himself the new leader of the Xiliens and used his powers to mind control the EDF's mutant soldiers, forcing them to attack the other humans. X then returned to the Xilien Mothership and ordered all of the kaiju under their control to be released.
When X learned that the Gotengo had survived and was en route to Antarctica, he sent Gigan to destroy it. The Gotengo freed Godzilla from Area G, and he quickly defeated Gigan. X proceeded to unleash countless monsters against Godzilla, including Zilla, Kamacuras, Kumonga, Anguirus, Rodan, King Caesar, Ebirah and Hedorah, all of which fell. X finally unleashed Monster X to battle Godzilla in Tokyo, while the modified Gigan battled Mothra. When the Xilien Mothership was boarded by the Gotengo, X had his underlings capture the crew and bring them to him. X taunted them, explaining that humans were "cattle" and that he planned to enslave and breed the humans so their mitochondria could be harvested as a food source for the Xiliens. He also revealed that the mutants on Earth were a result of humans and Xiliens interbreeding in the past. Some rare individuals called "Keizers" could obtain incredible powers from this combination of DNA, and X revealed that both he and Shinichi Ozaki were among them. X took control of Ozaki and had him attack his allies, but Miyuki Otonashi was able to free Ozaki from X's control using the Shobijin's sacred dagger. Ozaki and X then battled while Gordon, the rest of the crew, and the world leaders taken prisoner by the Xiliens fought their way back to the Gotengo. Channeling all of his hidden power, Ozaki defeated X, leaving him lying on the ground. X laughed, stating that he would not die alone as the ship began to self-destruct. Ozaki reached the Gotengo and escaped with the ship, while X screamed defiantly as the Mothership exploded with him in it.
- The visor that X is seen wearing in some scenes is a reference to the attire of the Xiliens in Invasion of Astro-Monster.
- X is capable of speaking fluent English as well as Japanese.