SPOILER WARNING: This article contains major spoilers for "O Brother, Where Art Thou?", the latest episode of Supergirl.

Lex Luthor has had an agent in Team Supergirl's midst all along. In "O Brother, Where Art Thou?", the latest episode of Supergirl, Eve Teschmacher revealed her true allegiance.

Since last season, Eve -- who went to Yale University and has a background in nuclear physics -- has helped Lena developed a formula to give humans superpowers. As it turns out, she was working with Lex all along, spying on Lena and reporting their progress to him.

RELATED: Supergirl: Superman Won't Return This Season

When Lena returned to the Luthor mansion after using the formula on James, she confronted her brother. She correctly ascertained that he had caused an emergency situation to force her to use it on James so that he knew it was sound before using it on himself, thus saving him from his Kryptonite poisoning. He confirmed her suspicious, then launched into the final phase of his plan. Otis Graves, who had been using an image inducer, revealed himself as the agent keeping watch over Lex and attacked her, then handcuffed her to Lex's wheelchair.

That's when the ultimate betrayal occurred. "Miss Teschmacher!" he cried, only for Eve to walk into the room carrying a gun. When Lena demanded to know how long she had been working with Lex, Eve simply replied, "Forever. It's nothing personal." She even shot a guard who walked into the room and accidentally discovered the tête-à-tête. Lex ordered Otis to knock Lena out, then told Eve to "stay here and take care of her," though it is unclear what that means at the time of writing. This enabled Lex to make his daring escape.

RELATED: Supergirl: Jon Cryer Is Lex Luthor Because of Superman IV

In Superman and Superman II, Eve is portrayed as Lex's ditzy personal assistant and occasional girlfriend, though she did help out the Man of Steel when push came to shove. Otis also appeared in those films as Lex's bumbling henchman. However, Supergirl's "O Brother, Where Art Thou?" brings the two characters together for the first time in Arrowverse history. Andrea Brooks has played Eve since the Season 2 premiere and has appeared in 21 episodes to date, but it looks like character has now stepped into a major new role with this dark turn.

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.