In “The Day the Earth Stood Still” (1951), the alien is named Klaatu.
Klaatu arrives on Earth in a flying saucer accompanied by the powerful robot Gort. He presents himself as a representative of an interplanetary civilization and warns humanity that its growing ability to wage nuclear war threatens the peace of other worlds. After being shot by a soldier, Klaatu escapes the hospital and adopts the alias “Mr. Carpenter” while living among ordinary people.
The character was played by Michael Rennie in the original film, directed by Robert Wise. Klaatu is not the robot: Gort is his imposing mechanical guardian. That distinction is the main source of confusion in this question, especially because Gort is associated with the film’s most memorable scenes.
Klaatu first appeared in Harry Bates’s 1940 story “Farewell to the Master.” The phrase “Klaatu barada nikto,” spoken in connection with Gort, became one of science fiction cinema’s best-known lines.