Lex Luthor
Alexander Joseph Luthor, or "Lex" Luthor as he is more famously known, is the principal antagonist of Superman. He first appeared in Action Comics #23 (April, 1940) and although his history has been retconned many times since then his current canonical origin is revealed in Man Of Steel #4 (August, 1986).
The original Luthor (who didn't have a first name) of the 1940s was one of many pulp fiction mad scientists who plotted to take over the world, or destroy it, through the use of various diabolical schemes. He donned disguises a few times, but generally he preferred to make himself known to the world as his master plans came to fruition...until he was foiled, time and time again, by the Man of Steel. He soon became Superman's greatest foe, the antithesis of everything Superman stood for; and even though his plans for world domination were repeatedly dashed, he always managed to get away (or escape from prison) to threaten the world time and time again. Luthor was revealed to be the master villain in the second Superman cliffhanger serial, Superman vs. Atom Man, though he never appeared on the classic 1950s Adventures of Superman TV series or in the original Paramount-Fleischer Superman cartoons. Actor Gene Hackman played the role of Lex Luthor in the 1978 movie Superman, and in two of its three sequels.
Over the years, Luthor came up with every conceivable plan to destroy Superman, and none worked (though one classic "imaginary story" from the 1960scalled The Death Of Superman has Luthor finally killing Superman with Kryptonite), but eventually the writers of his stories began to suffer from the same dilemma that they faced with Superman: How do you write an interesting story about an all-powerful being who is just a few steps shy of God? To freshen the rivalry between Luthor and Superman, they teamed him up with Brainiac on many occasions, and in the 1980s Luthor was given a more personal reason to hate Superman (he was the chosen hero of a distant planet that exploded, and Luthor blamed Superman for the disaster). In 1985, John Byrne's famous "re-boot" of Superman re-wrote the character of Lex Luthor from scratch, bringing him to the modern world and making him a villain that the 1980s would recognize: a corporate white-collar criminal. (The idea was originally suggested by Marv Wolfman.)
The current, "modern" history of Lex Luthor is as follows:
Starting out as an orphan in Metropolis' "suicide slums," Lex Luthor worked his way up to wealth and fame as a wealthy business tycoon. He is the CEO of LexCorp International, based out of Metropolis. He was the most powerful man in Metropolis until Superman arrived. When terrorists attacked a society gala aboard his yacht shortly after his arrival, Luthor observed Superman in action and then tried to hire him as a bodyguard. But when Luthor admitted that he'd expected the attack, Mayor Berkowitz deputized Superman to arrest Luthor for reckless endangerment. Luthor vowed to destroy Superman for this humiliation, and he has since devoted much time and energy to that goal; he is a man driven to be the best, having fought his way up from lowly beginnings by his own effort, and is resentful of how Superman was given his powers by random fate of birth. Superman survived every attempt but has never been able to prove Luthor's role in the attacks.
LexCorp's other experimentations with Kryptonite have had significant impact on Superman over the years, at one point inadvertantly transmuting all Kryptonite on Earth into iron when an attempt was made to use it as a power source. Later, to secure another piece of the now-rare Kryptonite, Luthor's men captured the Kryptonite-powered cyborg Metallo just before Metallo succeeded in killing Superman. Luthor suffered from a severe cancer in the 1990s, caused by the long-term radiation from a fragment of Metallo's Kryptonite set into his ring for protection from Superman. Kryptonite exposure had not previously been thought to be harmful to non-Kryptonian life forms.
Luthor's hand had to be amputated to prevent the cancer's spread, but unfortunately by then it had already metastatized; it was eventually determined that the disease was terminal. Luthor faked his own death shortly afterward by taking his private jet aloft and crashing it in some mountains, using this as cover for the transplant of his brain into a healthy clone of himself which he then passed off as his son and heir. Later, when his new body also grew terminally ill for other reasons, he had his brain again transplanted into a new cloned body that resumed the identity of the original Luthor.
Lex Luthor has cultivated a popular image as a great philanthropist; he has been instrumental in reverse-engineering alien technology for use in general consumer goods, upgrading Metropolis into a true "city of tomorrow," and when Gotham City was destroyed by an earthquake and then abandoned by the American government it was LexCorp that took up the massive task of rebuilding. He also played an instrumental role in assisting the Justice League in recharging the Sun during "The Final Night" crisis.
Lex became the 43rd president of the United States in 2000, winning the election on a platform of promoting technological progress (his first action as president was to take a proposed moratorium on fossil-based fuels to Congress in hopes of putting "a flying car in every garage"). He was assisted by the extreme unpopularity of the previous administration due to its mishandling of the Gotham City earthquake crisis; even given Luthor's tendancy towards covert supervillainy, he could well prove to be a better choice for the office.
Luthor is currently widowed, having killed the mother of his infant daughter, Lena.
In the current television series Smallville, Lex is teenaged Clark Kent's best friend. As a young boy, Lex was caught up in the explosion of baby Kal-el's rocket ship as it landed on Earth from Krypton. The explosion caused Lex to lose his hair. When Clark grew up, and was still learning how to deal with his super powers, he rescued Lex when Lex's car crashed through a bridge rail and went into the water. The two bonded, and Lex, living in Smallville as he runs his father's local business interests, considers Clark a "little brother". The first season of the show hints at possibilities of future enmity between them, possibly over the affections of Lana, who Lex initially tries to pair with Clark.