I’ve got a horrible, awful, no good, partially baked idea for Season 4
The black kryptonite splits Kara-- Red!Kara and our Kara. And Red!Kara gets it in her head that while Supergirl is National City's protector, Lena Luthor is its backbone. That by taking out Lena, National City's industry will falter, and the loss of technological genius would destabilize the entire nation enough to make a future attack more devastating.
So Red!Kara comes after Lena, and wounds her with a blast of heat vision. The DEO responds to the emergency alert (they've been monitoring the building since they found out about the Kryptonite) and finds Lena wounded in her office. Supergirl goes to help her, only for Lena to flinch away.
"Don't touch me," she pants, hand pressed tight against her side. "Y-you did this."
Lena looks to Alex, for protection but also to try and warn her. "Alex, it was her. You can't trust her."
Alex explains it's just shock, she's just confused. Supergirl couldn't have done this (one look at Kara's horrified face is confirmation enough). Lena doesn't calm until James arrives, and she tries to explain to him what happened, except he also is like "that makes no sense, it couldn't have been", and Lena finally just shuts up.
God forbid anyone believe the woman with a blast wound gaping in her side.
She's taken to the DEO for treatment. Alex at least bars Supergirl from the medbay (to keep Lena calm more than anything else). But Lena can't rest. She's on edge in this place where they're all loyal to Supergirl, where no one will listen or believe her.
Until Kara Danvers comes to visit.
At first, Lena barely looks at her. She sits quietly for a long moment, before asking what happened. Lena can barely contain her sneer.
"Why don't you ask your sister? Or James. Let them tell you the rantings of a raving Luthor."
Then suddenly, Lena's breath catches and her eyes fill with tears. Kara reaches for her hand. “Lena…”
"It's like I'm screaming into the void, but no one is listening! Is-- is this how Lex felt? Trying to warn a public who just refuses to hear it?"
"I believe you, Lena."
And Kara does. Truly. Because looking at Lena lying in a hospital bed, in pain, exhausted, and despairing-- there's no way she's making it up. Somehow, someway, Lena believes Supergirl attacked her.
Kara reaches out and takes Lena's hand in hers.
"I believe you."
Later, when Lena is finally resting, Kara leaves the medbay and runs into James on his way back in. Kara meets his gaze with dark eyes. "You need to cut ties," she tells him.
In an instant, James' hackles are up. "No. No way. I'm not breaking up with Lena."
"You have to cut ties with Supergirl."
Oh. That's not what James expects to hear. "Kara, we both know you didn't do this."
"No, I didn't. But Lena-- Lena needs us. She needs to know we believe her, that we're on her side."
Kara doesn't want to find out what might happen if Lena feels alone in the world. Everything Lillian promised would come to pass, and it would be just another Luthor against another Super, and Kara will not let that happen.
"She needs you now more than ever."
When Lena is released to go home, James stays with her. He's already apologized for reacting with disbelief rather than outrage, and it's enough for her let him help her, even if she doubts the truth of his change of heart.
Until one day she's just finished getting dressed after a shower when she stops in the doorway, and hears James speaking with someone. It doesn't take long to recognize Supergirl's voice, or for the anxious twist of her stomach at the sound of it.
"Please, James, I just need to talk to her."
"You're not welcome here. You need to leave."
"It wasn't me--"
"Lena doesn't lie. Not about this."
"I'm not saying she is. We've seen weirder things than doppelgangers. Anything she can tell us about what happened--"
"She'll share what she knows. But not with you. She doesn't trust you, and until we figure this out, neither do I."
Lena hears a rustle of movement, then the click of the balcony door opening. "If you need to contact Lena,” James finishes, “do it through the DEO."
Supergirl leaves without another word, and Lena emerges from the bedroom before James even turns around. She hovers, not quite closing the distance between them.
"How much of that did you hear?" he asks.
Lena shrugs. "Some. Enough."
James scrubs a hand over his jaw. "I know you could have handled it yourself, but with everything that's happened... I'm sorry if I overstepped."
Tears burn at the back of her eyes. "I didn’t think you believed me."
She hates the way her voice shakes, hates the way her lips pull before she can shove the tears back down. When James' arms close around her, she presses her face against his shoulder, finally accepting the comfort he's been trying to give all day.
"I don't know why she would do anything to hurt you," James tells her softly. "But you haven't lied to me yet, and that wound in your side is proof in itself. Until we know what's going on, Supergirl may be a danger and.... I'm not willing to risk you."
Lena feels the dam start to crack. "I--" Her throat catches, strangling her. "I don't want to turn into Lex. But if she's a threat, James..."
"You won't become your brother," James promises.
"How do you know that?"
"Because your brother was alone, Lena. And you're not alone."
Clenching her jaw against the tightness in her throat, Lena tries and fails to keep the tears from falling. They soak into the fabric of James' shirt, and stain her cheeks with damp betrayal.
"We'll get through this, I promise. Together."
Lena doesn't speak to Supergirl again-- except to warn her that she's manufactured more green Kryptonite. This time, the DEO makes no demands of forfeiture. Even Supergirl herself gives a solemn nod, as though accepting the threat Lena implies.
Lena doesn't know what to make of it.
Supergirl keeps her distance, but remains close as Kara Danvers. Their friendship recovers, and becomes as strong as it's ever been. Lena and James grow more comfortable, spending evenings and nights together in jeans and sweatpants, reading or cuddled up to watch television.
When Red!Kara tries again, it’s a rare moment when Lena is alone. She snatches Lena off her balcony and flies her up and up and up before dropping her.
Supergirl catches Lena, and brings her down to the L-Corp roof. Lena staggers away from her, scared and confused and panicking-- and Kara lets her go, focusing instead on the figure zooming towards them. Kara lunges for Lena moments before another heat vision blast can connect. Kara takes it on the cape, and it's then that Lena realizes something is off.
She looks over Kara's shoulder to see dark Supergirl hovering beyond the edge of the roof, and then looks back at Supergirl.
"Oddly enough… That makes more sense."