The Stranger, a name that doesn't come from the DC mythology. But usually, with these comic book shows, names like that are only used to cover up who they're actually playing. During the Superman & Lois pilot, he became the latest threat for Clark to take on. While being cryptic, The Stranger showed how much he hated Superman because he "destroyed" his world.
But towards the end of the pilot, viewers did get to discover who Parks is actually playing and how crucial he is to Superman & Lois season 1. As he removed his helmet when returning to his ship, the A.I. refers to him as Captain Luthor, revealing that another version of Superman's biggest enemy exists in the Arrowverse. While Jon Cryer's Lex hasn't stopped by yet, Clark will have a different Luthor to take on. But perhaps he isn't as villainous as some might have thought, as the second episode sheds a lot of light on this figure.
The latest episode, "Heritage", sees Luthor back and still persistent to prove to the world that the Last Son of Krypton is a threat. In the previous episode, Luthor made it clear that he knew a lot more about Superman than Clark realized, as he was referred to as Kal-El. But there is another character that Luthor tries to reach out to, ing General Sam Lane. While their encounter is brief, it is also a key scene that will be instrumental for Sam's storyline going forward. Luthor gives the General an S-shield with the number 7734 on the back of it.
However, a flashback shown later on reveals a lot more about this Luthor and why he hates Superman so much. As the A.I. even says, Earth-Prime's Kal-El isn't like the one that he had encountered before. Luthor then re fighting side-by-side with an army, including Sam. But what follows is a surprise as Hoechlin is seen wearing a black Superman costume, the same one from the Elseworlds crossover. However, this Superman is far worse than the one fans met in the 2018 crossover Clark kills everyone but Luthor with his heat vision. There's definitely more to tell, but just this moment alone establishes that there's a deadly Superman residing somewhere in the Arrowverse.
Superman & Lois then ends with Luthor stating that this version of the Man of Steel isn't like the Superman he fought, at least not yet. This paints Luthor in a very different light as he isn't just another evil Lex like the one Cryer plays. While his methods can certainly be questioned, it doesn't change the fact that in his mind, he's trying to save a world from Superman, believing that Kal-El is destined to turn on humanity. The Luthor that Parks plays gives the Arrowverse a new take on the iconic super-villain, while also setting up an evil Superman down the line for Superman & Lois.