Of all the legendary opponents to face Superman there ever were, none is as well known and has dogged in his determination as is Luthor. And although there has been a continuous publication history chronicling the development of this arch-adversary, hindsight has rendered this master manipulator as two and, in fact, three men in the multiverse that was.
The initial Luthor from Earth-Two appearing in the chronicles of our caped crusader was later named Alexei, a disaffected European with a grand intellect and a grandiose ego that knew no bounds. Attempting to conquer rival countries, the flame-haired felon eventually resolved to simply capture the city of his opposite number Kal-L's heart, Metropolis! With no relatives and apparently without an ounce of humanity, Alexei would harness any device to wipe out any who stood in his way of victory.
The more well known Luthor resided on Earth-One and, at the state, was a brilliant and benevolent young man who attempted to assist his good friend Superboy. That is, until an accident inadvertently caused by the Boy of Steel led to Luthor's own red hair falling out... turning him forevermore bald! This seemingly minor incident caused the mind of Lex to snap, and he ended up developing a variety of gadgets with the goal of humiliating the now grown Kal-El. In the interim, Luthor stumbled upon the alien world of Luxor... helped usher it into a modern technological age and became his hero, marrying one of its own and fathering a son. However, when he donned a new battlesuit to attack Superman high above that planet, he ended up destroying his adopted home. Finally, any morality left in Lex's heart was stripped away.
At one point, both Luthors swapped places and attacked the others' Superman... and then tried to conquer Earth-Three while destroying their own worlds with the aid of Ultraman.. until they were thwarted by their counterpart Alex Luthor. That world's sole superhero! Finally during the crisis, when Alex died alongside his planet due to anti-matter spilling over into his universe, he sent his son Alex who was then an infant to aid the heroes on the remaining universes that survived. When Alexei was destroyed by Lex's frequent collaborator Brainiac, and then Alex Junior disappeared into a pocket dimension after defeating the Anti-Monitor, only one Luthor remained to face the final effects of the Crisis on Infinite Earths on a reconsistuted composite Earth (in one published timeline, that is).