Lex Luthor is taking an increasingly central role in the Arrowverse's Crisis on Infinite Earths crossover - we break down why that is. Created by Jerry Siegel and Joe Shuster, Lex Luthor made his comic book debut on the pages of Action Comics #23. The Arrowverse's incarnation of Luthor made his debut in Supergirl season 4. Played by Jon Cryer, Lex served as the primary antagonist before he was shot dead by his sister, Lena Luthor. Ahead of the Crisis on Infinite Earths crossover event, however, Lex Luthor was resurrected by the Monitor.
Much to the chagrin of Supergirl and Batwoman, Luthor has since spent the crossover event alternating between helping the heroes and characteristically betraying them. One of his more villainous efforts saw him abscond with the Book of Destiny, which he used to jump between surviving universes to kill every version of Superman he could find. After succeeding multiple times in that endeavor, he encountered and was ridiculed by Tom Welling's Clark Kent from Smallville. Lex also inadvertently led a trio of heroes to Brandon Routh's Kingdom Come Superman - said to be one of the prophesized paragons who would ultimately defeat the Anti-Monitor. Unfortunately, the shocking cliffhanger of Crisis on Infinite Earths Part 3 revealed that Lex had given fate a rewrite and usurped this Superman's position in order to save himself.
In the original Marv Wolfman-penned comic book saga, Lex Luthor only plays a relatively minor role. Recruited by Brainiac, Earth-1's version of Lex served as one of his generals in an attempt to conquer the five remaining Earths. Brainiac has not featured in the CW's adaptation, however, and not a single Earth was left intact going into the final two Crisis on Infinite Earths installments, Because of that, it's unlikely that the crossover will follow entirely the same trajectory. Still, Lex Luthor's elevated role on the show is easy to understand. As one of DC Comics' most well-known and iconic villains, he could easily take Brainiac's place in a streamlined approach and even pursue the same agenda. As villainous as he is, the complete destruction of all that there is would be of no benefit to Lex. As such, he would, of course, lend his considerable genius to aiding the heroes. Even if simultaneously attempting to conquer whatever is ultimately left.
That genius-level intellect would also be a firm reason as to Lex's inclusion alongside The Flash, Martian Manhunter, et al. For all their considerable attributes and abilities, none possess quite the measure of intelligence and resourcefulness as Lex Luthor. Equally, having only heroes among the survivors could also run the risk of being a rather bland dynamic. Having Lex among them would add a level of humor, tension, and conflict to proceedings even when the Anti-Monitor and his forces are nowhere to be seen.
In the comics, the most prominent Luthor is Alexander Luthor Jr of Earth-3. Born to an infinitely more heroic version of Lex and Lois Lane, he is sent to Earth-1 in a fashion not dissimilar to Kal-El's exile from Krypton. Coming into his own superpowers, Alexander ultimately aids the heroes in defeating the Anti-Monitor before venturing to a "paradise dimension". While the Arrowverse's Lex Luthor possesses no such interdimensional powers, there's nothing to stop him using the Book of Destiny or other means to grant himself some. Equally, a new spin on trying to either create or access his own version of such a paradise dimension would make for an interesting subplot.
The Monitor previously stated that Lex Luthor had a role to play. Given that everything he foresaw was irrevocably altered following Oliver Queen's death and resurrection, however, the nature of that role is now firmly up in the air. There is still much for Lex to do, however, especially since the part of his death with The Monitor pertaining to getting vengeance against Lena remains unknown. Whatever the case, though, Lex is sure to make for a wildly entertaining wild card when the Arrowverse returns and as Crisis on Infinite Earths concludes.