SPOILER WARNING: This article contains major spoilers for "House of L," the latest episode of Supergirl.
Supergirl's Red Daughter wasn't as blank a slate as Kasnia's military believed. When she arrived at their compound, she only remembered one word -- or, rather, one name: Alex. So, when Lex Luthor came to her and introduced himself, she forged a bond with him that could prove to be Kara's undoing.
In "House of L," the latest episode, Lex's secret history with Red Daughter was unveiled. As it turns out, he has had a hand in her training since she arrived in Kasnia. When the Kasnian military realized she was Kryptonian, they brought in Lex, who used bribery and threats to take a little vacation from his prison stay.
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Lex arrived during Red Daughter's moment of distress. After she picked up a young boy's cry for help through her super-hearing, she abandoned her cell and came to his aid. In the process of saving the boy Mikhail, she killed a group of thieves who had broken into the young boy's home. Lex and the Kasian officers discovered her huddled in the corner, crying and shielding Mikhail with her body.
Cautious as ever, Lex approached her with kindness. "Hello. He has nothing to fear and neither do you," he said. "My name is Lex."
"Alex?" Red Daughter asked, to which Lex responded, "Sure." Since that name is all she can remember, she immediately trusted him and bonded to him. This, of course, opened her up to be manipulated by Lex. From there, he would go on to shape her into a weapon loyal only to himself.
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Naturally, this moment begs comparison to Batman v Superman: Dawn of Justice's infamous "Martha" scene. In the DC Extended Universe film, which is in no way connected to the Arrowverse, the Man of Steel and the Dark Knight come to blows over a misunderstanding constructed by Lex Luthor. However, just before Batman could strike the killing blow, Superman choked out his mother's name: Martha. Since Batman's name was also named mother, the namedrop distressed him, allowing Superman to explain that Luthor was holding his mother hostage. This exchange humanized Superman in Batman's eyes, opening the door for them to work together to bring Luthor down.
Likewise, Supergirl and Red Daughter have a person named Alex in common with one another. Unlike Batman v Superman, though, having this name in common further divides them. Because Red Daughter has a positive impression of Alex Danvers but no memory of her, Lex was able to take advantage of her by claiming to be "her" Alex, which leads her down a dark path. Hopefully, Supergirl and Red Daughter will realize they have more in common sooner rather than later.
Airing Sundays at 8 p.m. ET/PT on The CW, Supergirl stars Melissa Benoist as Kara Danvers, David Harewood as Martian Manhunter, Mehcad Brooks as Jimmy Olsen and Chyler Leigh as Alex Danvers.