Klaatu

"Klaatu barada nikto."

- Klaatu's phrase used to stop Gort.

Origin
Klaatu arrives in a Flying Saucer in Washington, DC, wearing a silver spacesuit and accompanied by a large humanoid robot, Gort. Klaatu arrives as a peaceful ambassador from an extraterrestrial confederation, but when he presents a harmless device he is shot by one of the soldiers that form his rather apprehensive welcoming committee. Klaatu explains the device was a gift to the President and could have been used to study life on other planets. To protect Klaatu, Gort vaporizes all of the surrounding weaponry.

Klaatu is taken to a hospital, where he quickly recovers. On learning that the government does not expect other nations to be willing to come to meet him, he goes out to meet the common people, using the surname "Carpenter," the name found on the clothes and suit case he borrows, as a pseudonym. He takes up residence in a boarding house and is shown the town by a young boy named Bobby Benson who stays there with his mother, Helen Benson, and drops by the home of Professor Barnhardt, a brilliant scientist, who is absent.

Klaatu/Carpenter leaves a "calling card" by marking Barnhardt's calculations as a teacher would mark a student's. When Klaatu/Carpenter is brought later by an army driver, Barnhardt discusses the situation with Klaatu, agrees to summon scientists from all nations, and asks Klaatu for a show of power, thus exhibiting that the extraterrestrial alliance controls enormous power that would threaten Earth if Earth threatens the peace in space. Klaatu, in reply, neutralizes electric power all over the world (with some notable exceptions, such as hospitals and airplanes in flight) for 30 minutes. Klaatu's action is taken to be an act of war. Betrayed by Mrs. Benson's boyfriend, he is hunted down and fatally shot.

Because Gort's purpose is to prevent aggression and could "destroy the Earth" if anything untoward happens to him, Klaatu has told Mrs. Benson to deliver the phrase that will cancel the attack. Helen reaches the robot, uttering "Gort! Klaatu barada nikto!" barely in time to save her own life, whereupon Gort aborts his attack, retrieves Klaatu's body, transports him to the saucer, and revives him from death - although Klaatu explains to Helen that this procedure is only temporary.

After Klaatu is revived, he steps out of the saucer, giving the scientists the following ultimatum: Earth can either decide not to threaten aggression on other planets and join other spacefaring worlds—a peace ensured by a massive deterrent force, consisting of robots similar to Gort—or else "this Earth of yours will be reduced to a burned-out cinder". Declaring "the decision lies with you", Klaatu then departs.

Keanu Reeves as Klaatu
The 2008 remake of The Day the Earth Stood Still features Keanu Reeves in the role of Klaatu and Jennifer Connelly as Helen Benson. While the original Klaatu was simply of a near-human species, in the remake it is stated that Klaatu's race is not remotely human, so that Klaatu's biology was radically altered to give him a human form acquired in 1928. When asked if he was human, Klaatu merely responds that "this body is human" and that "this body feels pain" when asked about that. No description of the race is given, other than that it is "different" from humans and the look "would only frighten you". He is captured by the military but escapes with help from Helen. He meets with a man named Mr. Wu (actually another alien of Klaatu's race in disguise, who has been living on Earth) to discuss whether or not humanity should be destroyed and they both agree that it needs to be done for the good of the planet, although Wu is unwilling to leave due to his newfound love for the humans. Klaatu orders smaller spheres—previously hidden on Earth—to begin taking animal species off the planet (like an ark) and activates Gort to destroy humanity. After spending time with Helen and her stepson Jacob, as well as talking with Professor Barnhart, Klaatu decides to try to stop humanity's destruction. After saving Jacob and his mother from Gort, Klaatu sacrifices his physical form to stop Gort, setting off a massive electromagnetic pulse before his body is destroyed. The EMP destroys Gort, saving humanity, but disables all electric technology in the world to give them the first step toward saving the earth. His spaceship/sphere then departs Earth after the pulse. It is unclear whether Klaatu survived or not; his human body was destroyed, but he indicated throughout the movie that he was merely residing in it and controlling it, so its destruction may not have killed him.

There are many differences between this version of Klaatu and the original, the most obvious being his expression of emotions and view of humanity. Reeves' Klaatu shows very little emotion, and explains this is because Klaatu "is an alien entity contained in a human body, and when he is looking out of that body, he is just looking out." The filmmakers thought that it would not make sense for an alien being to act as human as Rennie's Klaatu did, and so they "decided that in the beginning he should just be very different. That involved having no natural human gestures or behavioral signs but appearing more flat and expressionless. He has a concentrated way of seeing the world."

Klaatu has a very cynical view of humans, being very strict with them. This is best shown when he is talking with Secretary of Defense Regina Jackson (Kathy Bates). He is more concerned about the Earth's botanic and animal species than humanity, almost caring nothing about them. Another, less obvious, difference is his knowledge of the planet. The original Klaatu knew nothing of Earth, and he spent most of the film learning about it to decide whether or not it should be destroyed. In the later film, another member of Klaatu's race had been sent to Earth decades earlier to judge the planet and had remained living among humanity for 70 years. From the other alien's report and his own observations, Klaatu knew enough to conclude that humanity should be destroyed. However, both Klaatu and the alien sent before him, eventually realized humanity had the potential for good and should be preserved.