Ao3 story (Why would you ever kiss me)

Part 1

El was horrified when she saw her brother's eyes roll back as he entered a trance like state. Vecna had finally found him, after hiding him for so long, he was being taken from her. The brother that she never had, the brother that she deserved. He was selfless and kind, always putting others' needs before his own. He didn't deserve to be taken.
El entered Will's mind, waiting for the familiar wash of darkness to enter her vision. There was only one way to protect Will from Vecna, to keep him from becoming possessed again. Without hesitation, she tied their minds together, not knowing what that choice would ultimately lead to.

—-
Will woke up with a start, his eyes searching the bedroom in a frenzy, his body tremoring with every flash of adrenaline that went through his body. 001 had found him. He looked about, nothing really seeming amiss. It wasn't until he saw someone sleeping next to him that he considered the possibility that it had only been a dream. Only, looking closer it was like he was staring in the mirror.

Will Byers, himself, was sleeping next to him, a dried smear of blood beneath his nose. Will looked at his hands, noticing how they seemed slighter than before, not the hands that used to be smeared with colorful pigment after hours of painting. He reached for his head, only to feel a fine buzz instead of the straight strands he was used to. I guess he was due for a haircut anyways?

The panic began to set in as he scrambled out of bed and hit the ground, his legs felt like jelly. He was a head shorter than normal, nothing felt right. He stood up on wobbly knees, holding out his measly arms for balance. Everything was wrong. He floundered to find a mirror and when he finally did he went stock still. El's chocolate brown eyes stared back at him, wide in an expression that was familiar, replacing the muted hazel color that he was used to.
His stomach turned over, this had to be a nightmare, a hallucination, something Vecna had conjured up to taunt him. He rushed over to his body, relieved to see the steady rise and fall of its chest.

He gripped his body's shoulder and shook it in a panic hoping that this would wake himself up and end this strange astral projection that he was experiencing. All he got was a muffled groan in response. His body wasn't just alive, it was occupied. Vecna. His fear sparked and he took a step back as he waited for death.

His body finally sat up and looked around, rubbing its eyes and yawning. When they met gazes, Will's fear dissipated. This couldn't have been Vecna.

"It worked." His body said. It was strange hearing the natural tenor of his voice in person. In his head his voice didn't sound like that at all. Hearing his voice himself, well, he sounded almost like a grown up.

"What's going on?" Will asked, shocked to hear a feminine voice exit his mouth. He touched his throat, noticing the absence of his Adam's apple. He was El. He was a girl. Oh, god.

"Will?" His body said, an incredulous look on its face.

"Yeah." He stared into his eyes, that's when realization struck him. "El?"
His body stood up and approached him and he found it strange that he had to look up slightly to meet his own eyes. They put their palms against each other and felt the first semblance of true familiarity in the action. Fear turned to curiosity.
"What did you do?" Will asked, pulling away from her. He had to turn away, looking at himself made him feel like he had gotten a hold of his brother's stash of 'Purple Palm Tree Delight'.
"Vecna almost got you Will. If that happened it would have been game over, and I don't think I could've lived with myself if something happened to you."
He gave her an appreciative glance before looking away in discomfort. "Thank you. But that doesn't really explain this," he said, gesturing to his entire body that was not his.
"Vecna's never going to stop looking for you Will, you're the only one who could be a fitting host for him. When he got a hold of you last night, I did the only thing I could." She took a deep breath and sat down on the edge of the bed. "I went into your mind and took permanent residence, but the only way for my body to live and for you to truly be safe...I guess I switched our bodies."