Short answers, pulled from the story.
George Lucas created Palpatine as part of the Star Wars franchise. His earliest notes discussed a line of corrupt emperors rather than a single ruler, and the name Palpatine first appeared in the prologue of Alan Dean Foster's 1976 novelization of the original Star Wars film.
Ian McDiarmid portrayed Palpatine in all five films in which the character physically appears, beginning with Return of the Jedi in 1983. In The Empire Strikes Back, the character was originally played by Marjorie Eaton with voice work by Clive Revill; McDiarmid replaced that version in the 2004 DVD Special Edition release.
George Lucas drew on Julius Caesar, Napoleon Bonaparte, Alberto Fujimori, Ferdinand Marcos Sr., Adolf Hitler, and Richard Nixon when developing Palpatine. Nixon's presidency prompted Lucas to think about how democracies transition into dictatorships without being overtly overthrown.
Palpatine's Sith name is Darth Sidious. It was first used in the novelization of the 1999 film The Phantom Menace.
Palpatine first appeared in the 1980 film The Empire Strikes Back, where he appeared as a hologram to inform Darth Vader that Luke Skywalker had become a threat to the Empire. He was referred to only as "the Emperor" in the original trilogy.
Director J. J. Abrams stated that Palpatine's absence from the sequel trilogy would have been conspicuous given his importance across all nine chapters of the Skywalker Saga. Rae Carson's novelization of The Rise of Skywalker reveals that Palpatine discovered the secret to immortality from his former master, Darth Plagueis, allowing him to survive his death in Return of the Jedi.