A/N: Just a theory I had about the Ruby switch. The personalities of the different versions seemed too different to be the same, so here's a rough idea of what I think happened. Please let me know what you think.
Ruby screamed. Her voice ripped out of her lungs as fresh blood poured out of the gashes in her vessel. There was a sigil carved into her sternum, a trident of sorts, that she quickly found out captured her essence and held her, immobile inside her human body. Twisting a demon in their natural state can be tricky. A human form in Hell shackles, strapped down to a gurney, being forced to relive excruciating pain over and over... Pains that no demon has actually ever felt before, Ruby knew she wasn't in for the royal spa treatment. The blond hair on her vessel mottled with coagulating blood, she turned her head and attempted to blink the brimstone and ashes out of her eyes. She was in a room that looked like a decrepit morgue, dark but for the one overhanging florescent light. Lillith appeared before her, gloved hand holding a scalpel, white eyes leering over a gore-splattered hospital mask. The rest of her body was hard to discern from the storm of black smoke that billowed around her. Only her face and arms were visible. Ruby heard Lillith's voice in her ear, in her mind, surrounding her completely. The accent was slick, sickeningly sweet. The voice of a child. "You've been a very very naughty girl, Ruby. I'm afraid..." she took a deep, dramatic breath, exhaling the scent of burning sulphur, "I'm afraid you're just going to have to remain in time-out until you've learned your lesson."
"Please, no more..." she whispered, shutting her eyes. She jumped when she felt Lillith's hand touch her face, thumbnail scratching softly at her jawbone. She shook, feeling her other hand, the one holding the scalpel, inch closer to her exposed midriff. She laid the cold scalpel against Ruby's skin, then barely pressed in, a drop of blood beading at the site. Ruby whimpered. "Can you at least tell me WHY you decided to help those..." she grunted in disgust "PATHETIC Winchesters? Why you turned against your kind to give them a meager advantage?" Lillith actually sounded surprised. Demons had never been like Ruby was, more in touch with humanity, less malicious for the sake of being malicious. Ruby was an odd duck. Ruby flashed her black eyes at Lillith, her mouth curling angrily. "You can go fuck yourself," she said coolly. Lillith simply smiled, and thrust in, the scalpel pushing through skin, muscle and organs easily, her hand sliding into the wound up to the forearm. Ruby screamed, tendons on her neck standing out in agony. Her blood splashed, warm and heavy, over the side of her body and the table to the floor drain below.
"Hmmm," Lillith moved the mask aside, pursing her lips. "Not quite the answer I was looking for. No matter. You probably didn't know this," she sneered at Ruby's shaking form, leaning in to whisper in her ear conspiratorially all the while shoving her arm into Ruby's body further, "but you haven't done all wrong in helping them. We now have an easy in for my backup plan." Lillith smiled at her, like she was a proud mother. "You've created the perfect cover to break the seals. Good for you!"
She pulled back after licking the shell of Ruby's ear in one quick swipe. Ruby had stopped screaming in lieu of blank, wide-eyed staring and raspy, shuddering breaths. Lillith yanked her arm out of the gaping hole in Ruby's guts and before she could register it, her vessel was as good as new. Lillith stood, drawing herself to incredible heights above Ruby's gurney table. "You've been down here longer than you think, silly." Ruby's eyes widened in confusion. She knew time was different down here, but how long? How many years of demons tearing and burning her flesh? Lillith simply smiled at her and began walking slowly around the table, trailing her fingers along the cold metal edge. "You've been replaced, my dear. Some demon named Ades who is faithful to me. And the best part? Those mindless buffoons can't even tell the difference! Sam was just so eager to have his Ruby back. She has taken a new vessel- some brunette corpse. When Sam is ready, they'll kill me and release our father sooner with 'your' help." Lillith giggled, a high childish sound. At this, Ruby went slack, her black eyes casting around in shock. Lillith noticed, stopping at the foot of the gurney once more. Her grin grew monstrous, face warping and innumerable rows of glinting teeth shining in the bright light. "You didn't even know." She whispered gleefully. Ruby sobbed as Lillith's laughter filled the room, harsh and metallic-sounding.