How could one mistake have cost him so much? How could he have let all this happen, and just because of a few careless words? That's why she left.
He hadn't even been thinking. He'd just reacted. He was sure that she'd be fine, like she somehow always was. She would storm off, and so would he. Then less than thirty minutes later he would knock on the cabin's door and run inside to apologize frantically. She would hug him. He would say 'Sorry' more times than he could count. That was how this was supposed to end.
That was not how it ended.
He rushed to the cabin, and knocked on the door, just like he was supposed to. He ran inside as soon as Hopper opened it, expecting to see El on the couch. She should have been watching a soap, or something equally cheesy. Instead, Hopper told him that she wasn't back yet.
That was the moment the creeping sensation that something bad had happened began. His mind began to run through all sorts of scenarios, and yeah, him and El were both seventeen. He shouldn't be so worried about her. During the rare times they did fight (And always about something stupid), she would sometimes take a walk.
Mike left, with a promise that he'd find her. He always did.
So began the search. It only took around a hour for him to realize that she wasn't anywhere she'd normally be. She wasn't at the library. She wasn't with Max. She wasn't in the library. She wasn't at the Byers'. He was almost ready to give up and head home so he could call Hopper when he saw it.
Sitting right there, parked on the side of the street, was a white van. One with the lab's logo on the side.
This sent Mike into a panic. They couldn't be here. El was hidden. She had a secret identity. No one knew her real name. They couldn't know she was still here. Once again, the feeling that something bad was going on worsened. That van had something to do with what was going on, and something to do with wherever El was.
He hurried home and phoned they all got the news, everyone agreed to meet just one corner over, that way whoever was in the van wouldn't see them arrive. It didn't take long before everyone was there. Hopper, Steve, Dustin, Lucas, Max, and him. The only one missing was El, and they would find her. They had to.
All of them marched up to the van, and Dustin got close enough to peer in. All he could say was that no one was in the front seat. That was when they went for the back. Hopper used a metal tool he'd brought with to pry the doors open, and what greeted them was horrible. Two men, obviously with the government, were sitting in the back. They were both slumped forward. If he ignored the unnatural whiteness of the bodies, they could be asleep.
Mike didn't know who these men were, but he knew one thing for sure now. El was in danger. They'd tried to grab her, and she'd killed them. But where was she?
Everyone went to their houses, mostly to look for her, but Mike knew she wouldn't be at his house, so he went to the quarry. She might be hiding there, waiting for him to come. He holds onto this hope.
When he arrives, there's not even a hint she's anywhere nearby. Still, Mike looks around. For all he knows, she could still be mad at him from their fight. He doesn't think she really will be, but anything that keeps his fear away is a good thought.
By the time he's sure she's not anywhere near the quarry, it's almost dark. At this point, he's at that place, the one where they pulled Will's fake out of the water. The cliff that he once jumped off of is high above him.
Just as he's about to leave, a light shines from that cliff. He watches quietly. It's probably just some kids from school or something, but it could be anyone. Better to not get caught here this late.
So he stays put and watches as a man with white hair walks close to the edge. Then, a girl with curly hair follows. Mike's breath hitches when he recognizes her. It's El. But what is she doing with Brenner?
(Because that's clearly who the man is now that he looks closer.)
Then, something happens. El's head twitches, a movement he can see, even from this far away, and Brenner goes plummeting off the cliff, but not before he grabs El's ankle. She seems calm, but Mike isn't.
A horrified scream rips past his lips. El freezes, and so, it seems, does time. She spots him, and her eyes widen before a small smile lights up her face.
Brenner's hands seem to release from El's ankle on their own, and he hits the water with a splash. El's decent slows until she's floating above the water. Then she floats closer to him.
When she arrives at the shore her nose and ears are leaking blood, but he doesn't care as he hugs her and whispers apologies over and over. She doesn't say anything. All she does is hug him. Then, as her grip on him begins to loosen he sees she's bleeding somewhere else. Somewhere from her stomach. He'd thought he heard a bang.
El gives him a weak smile, then her body slumps. At this point, Mike has tears running down his face silently. He's hoping someone comes soon, because he can't leave her here just to find a phone. He has to stop the bleeding. He tears his jacket off and presses it against the wound.
And just as he's about to start whispering, telling her it's going to be okay, she puts a finger up to his lips.
"Mike," she said, voice filled with so many types of pain, happiness, and love. "It is okay."
Then she pulls herself up, and kisses him gently.
When both of them pull away, El has a sort of self-satisfied smirk on her face. But it only stays for a moment before her face smooths out in peace. She stops breathing. She doesn't ever start again.
I am so, so sorry! I just had this idea, and I knew it was depressing, but it seemed interesting, so here it is. If it helps, I was going to be much meaner and make it so the last thing that Mike ever did to El was fight with her, but I grew a half-heart at the last moment.