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The day they're at the Lockwood house, she proposes a code. They are allies now, they both need each other to defeat Klaus. He had agreed, wondering if she had perhaps seen too many spy movies.
He memorized and practiced the code anyway.
I trust you, she had reminded him in code, as he took her like a lamb for his brother to slaughter.
He didn't spend too much time thinking of the code while he was daggered.
When Klaus discovers he needs the doppelganger's blood, and that she's alive, in Mystic Falls, he undaggers Elijah. He tells his older brother that he needs to get blood from her for his hybrids. If he fails, he'll be daggered again.
It's a test of trust, he recognizes. His brother has cheerfully reminded him that he could kill Elena's loved ones and simply take her. He's trying to keep a low profile, however, and thinks the newest doppelganger will trust Elijah more than she'll trust him.
If she doesn't, his brother tells him, he can compel her. He wants his brother with him on this, and he knows how much his brother values free will. They could be a team again. He wants it. But he doesn't say it in a sentimental way. It wouldn't do for his brother to see him as weak.
Elijah agrees, admitting that he missed the relationship they once shared. Klaus grins, showing all of his teeth.
But when Klaus is sleeping, he knows he needs to contact Elena.
He could run. He could leave, let Mystic Falls try and deal with his brother. He doesn't.
When he shows up at Elena's doorstep, she freezes. She tells him, brushing something off her left leg, that she hates him. Her eyes meet his when her attention is back to him, and she looks him dead in the eye and says she never wants to see him again. He betrayed them. He betrayed her. She died, actually died, for nothing.
She almost closes the door before he tells her calmly, that he will be back to get some of her blood the next day. He's working with the brother he betrayed her for, he implies. Her eyes are cold and she shuts the door. He wonders why her words stung a little bit-and then his eyes widen.
Her heartbeat had jumped, he had assumed it was fear-no, she was lying. Brushing her left leg-code that she was lying.
He has no doubt she spotted Klaus's spies, his brother has assigned a few to her and Elijah. So they're always monitored.
It's the first time he's ever been happy to hear the words "I hate you."
He thinks back at her other codes, how she acted. She wants to meet him at midnight, if he can make it.
He convinces his brother of his loyalty by telling him he has an appointment to take Elena's blood, and a few other things that night. He plans to ask to get the spies off of him and his still human ally for the night.
His baby brother agrees after a few bottles of wine from the prohibition era, and reminiscing. He falls asleep after several bottles, and he has tried to kill his brother. He has wanted to rip him apart for quite some time, he took Rebekah, Kol and Finn away. But he just picks him up, and tucks him into his bed. Klaus opens a bleary eye, and his older brother just tells him goodnight. The hybrid nods, and his eyes close. He waits to leave the house when he can tell he's asleep. Only a few minutes from midnight.
He's almost a minute late, but Elena looks relieved when he sees her peeking through the curtains.
They make a plan, that night.
It's not the best plan, it's not good for either of them. He's asking a lot of her, and she's asking him to help her keep everyone else safe from his brother. He'll give her some of his blood after she gives a pint. He apologizes, but they don't have much time for a while, to make a plan. She might have to give blood more than once, but he promises to keep her safe too. They refresh each other on code.
He gives her a time for the next day, for her blood withdrawal.
She taps out the word trust in Morse code against his shoulder, as his brother's eyes and ears leave momentarily with her blood. He gives her blood, they took too much. He taps it against her shoulder as she takes his blood, a confirmation.
When his bite closes, she lets go of his arm, and he offers his handkerchief. He's sure she doesn't want to explain the blood on her lips to her family when she gets home.
She only gives blood once, only takes his once. His family is found and awakened before Klaus can try to get more, four weeks later.