I've attempted this before but it sucked - it really really sucked :/. I'm hoping this one ends up better. This is pre-Sam says yes to Lucifer and after but with a twist... 50/50 (more or less) canon and the time line is a bit off... I am trying different writing styles but my word flow is unique to my personality so hope you all enjoy.
Sam and Dean Winchester were driving in their... er... Dean's black '67 black Impala. In the mist of the angels, demon slayings, more drama that two brothers should never have to deal with on their own, they were unaware that they were being followed. Their follower was very careful not to be detected by them or the angel. To keep distance and observe for the best possible moment to expose. That is what the follower was taught.
The night that Lucifer killed all the pagan gods except for Kali in the hotel, he walked to the side, by the alley. There he saw the follower on a black street bike, "How long do you intend on hiding in the shadows, girl?" The follower came out of the shadows, being seen by the street lights above. She was average height but he easily towered her. Pale skin, thin, long black hair in twin braids. Her black sunglasses covered her eyes.
"So you sense me, " She smiled, "I'm glad to meet you first, Lucifer. Let's chat."
The Winchesters were preoccupied finding the rings of the horse men. She followed them as far as she could. She was very much involved in everything they did but from afar. Her presence was being sensed by their main enemies. No one but Lucifer knew who she was and she preferred to keep it that way. She needed that air of mystery around her for a while longer, no one needed to know who she was just yet.
It was when she heard of Adam that made the whole thing more interesting into her reveal. She called for Lucifer to a unknown location outside of the town the Winchesters were located at the time, sleeping away the fatigue of battle.
"So there can be substitutes as long as there is Winchester blood. Dean shall be spared and as for Sam..."
Lucifer eyed her, "You propose to take his place?"
She looked at him, sunglasses kept her eyes hidden, "Ideally. Sam and Dean have bigger plans than this prize fight over humanity where we're screwed either way. I can't risk the chances of them not being 100% for when that time comes. Adam has already said yes to Michael and I am willing to say yes to you."
Lucifer couldn't take her seriously; why would she be willing to sacrifice herself for someone she's never met. She cocked her head to the side and chuckled, "I can read it all over your face. You don't believe in my selflessness. I'm not misleading you, Lucifer. Consider it an exchange of destiny. You know who I am, which is why you were able to sense me. Instead of that unwilling giant, you'd get 'ol willing little me. Sounds fair, doesn't it?"
Through the dark lenses he could see her eyes and could smell her determination and honesty, "Deal." She licked lips and smiled, "Great. Oh, here's the final kicker to our deal: tell no one. It's been entertaining watching them figure out how Sam will say no to you and really, it's good to be of help while I still can be. When the time is right, come find me."
"So it's almost time." Sam said quietly, lying on the motel single bed. Dean was sitting on the other single sized bed beside Sam's, facing him. Castiel stands at the foot of Sam's bed, his arms folded, "You are not going to say yes." The elder brother looked at him for an answer.
"Of course I'm not!" Sam shouted. He threw a pillow over his face. Dean wasn't so convinced and crossed his arms as well, "You don't seem very convincing, Sammy. There is no one else who can take your place and sooner or later, he'll figure out how to nudge you enough to say yes." Castiel lowered his head, "I'm not so sure about that. Michael skipped over Dean and chose Adam for his vessel because he was half Winchester. What if Lucifer can find someone else suitable to take Sam's place?"
"What? Like another sibling we don't know about?" Dean grunted, "Impossible." Sam sat up on the bed, "Is it? I mean, we don't know what Dad did on those trips. We don't know he was with." "It's also possible that the angels and demons are in the dark of any other siblings you two may have." Castiel added on.
Through a tip, the brothers gathered at Stull Cemetery outside of Lawrence, Kansas. There they saw Adam and the young woman they never saw before. The Winchesters got out of the car. Adam and the woman looked at the brothers, she smiled with a bit of malice, "We wondered if you'd both show up for the show." Michael looked at her, "Stop toying, Lucifer. Fight me and get this over with."
"I'm sure we don't really have to fight? Imagine how many of Daddy's imperfections would die because you want to be his shadow." Lucifer tried to reason with Michael. Michael didn't buy it. He took a step back and narrowed his eyes, "I don't have all day, Lucifer. Fight me or die."
"Lucifer?!" Sam and Dean looked at each other. Dean whispered to Sam, "Couldn't have chosen a better vessel than a goth girl." Lucifer eyed Dean, "Shut your mouth. She may not been my chosen vessel but she was more than willing to take his place as Adam was yours." They watched as the vessels of Michael and Lucifer battled it out. Castiel appeared and stood off to Sam's side, "So the rumors are true. There was another Winchester."
"What?!" Dean, Sam, and even Adam were confused and looked at the female Lucifer. By that time she was out of breath, bloody, and clothes torn from the fight. She struggled to breathe, "I rather not fight you at all, Michael, but since you were so dead set, another vessel presented herself to me. I promised I wouldn't leave her a mess, but I never said that she'd survive the fight." Sam threw the rings on the ground and chanted for the pit to open. Lucifer fled out of the girl and Castiel was able to save her in time. The brothers watched as Adam was sucked in and the pit closed. The girl wasn't breathing.
Castiel healed her the best he could, it would be a while before she woke up. They took off her sunglasses, not wanting her to break them as she was out. After a couple of days, she woke up in a motel room, fully clothed, and her hair seemed combed, "Where am I?" They finally saw her dark green olive eyes, which they mentally agreed were beautiful.
"Our questions first. Who are you?" Dean asked, looking over her.
"How were you able to contain Lucifer in your body?" Sam asked.
She sat up and pushed herself to lie against the front of the bed, "My name is Cryssi and... I'm your sister. I have been following you two for a while now. Even helped you along the way, and not a single thank you."
Sam peered at her, "For how long exactly? I swore I've seen you around before."
Cryssi sighed, "Um... I've been following Dean for a while now and then the beginning of your adventures together - until now obviously. Lucifer was able to sense me because we share half a blood line. I only said yes because I know you two have bigger plans than to let two arch angels use you as avatars in a fighting game. Heh, interesting isn't it?"
Dean got up, "We're in Kansas. Although you've been unconscious since you let Lucifer take your body, we'll let you rest and then we'll pry you for answers. Don't think about escaping, Castiel here will watch you all day and all night if he has to."
"All night? Isn't that rather... oh I don't know... disturbing?" Cryssi asked, looking at the attractive male in the trench coat. Dean shrugged, "We all get used to it." She looked at Castiel, imagining doing dirty things to him, and smiled, "On second thoughts, I don't mind at all."
Sam looked at her face and whispered to Dean, "Isn't that cute, she's totally crushing on Cas."
The next day, Cryssi realized that Lucifer didn't bring along her duffel bag with her belongings, "So funny story. Lucifer didn't bring along my bag, which has, like... everything in it. So... wanna go clothes shopping?"
Sam and Dean looked at her, realizing they didn't really have a choice, they agreed. She picked out clothes that didn't seem to really go together: goth and girly. After shopping, she went into the bathroom and changed into blue jeans and a pale pink fitted shirt. Her hair was in loose waves down her back. She wore a pink wide head band, "Don't I look pretty?"
The guys nodded slowly and she was able to sense what was on their minds. She smiled, "What? Did you think I was Goth? Nah. I just like the clothes. My style doesn't define me, just what ever looks good."
They went back to the motel. She bit her lip as she looked out the window at the impala, "Do you guys think I can tag along for a while? I... I would like to hunt with you two instead of watching from afar."
The brothers looked at each other and nodded. Dean looked at Cryssi, "Fine, but on one condition."
"Absolutely... what?" Cryssi asked only to hesitate.
"No secrets. You have to tell us everything. If you hunt with us, you'd have to live with us, and if you're keeping secrets, we won't be able to trust you." Dean answered. Cryssi agreed, "Deal. So it was..."
Cryssi told them what they needed to know, keeping more personal stuff to herself for now: how she became a witch and founding out she was a Winchester. She told them how she'd follow them and watch from afar, bummed out she couldn't help them directly without a chance of blowing her cover. She told them why she agreed to be Lucifer's vessel - she didn't expect to come out alive. She assured them that her spells were used for good, as she had no interest on hexes. She even prepared them anti-hex satchels, which she presented from her purse, "I hoped to give them to you someday. Open them if you don't believe me. I'm sure Castiel can determine if it's good magic from bad."
They did just that and Castiel confirmed her truth. Dean gave in and let her join them. They figured having a girl around would make things interesting for them, not knowing what they really signed on with. Cryssi sat in the middle of the back seat, happy to be between her older brothers. Castiel often sat to her right side and would let her lie against him when she'd get tired from the long car rides.
"She's going to come in handy more than we'd ever know."
Dean looked at Sam, "You think so?"
Sam nodded, "She's not the kind of witch we're used to. She's sincere and could be a great asset against evil."
"I dunno, Sam. At the same time, I don't want to throw doubt where it's not needed. We'll keep an eye on her for now and see how well she can fight.