So to make this make sense, Sam is 28 in Season 1, making him 16 or 17 when Cassandra was born. She is 12 at the start of season 1 and had just turned 15 when she went to hell to save Sam. This takes place in season 5.

Name: Cassandra Aurora 'Rory' Winchester
Nicknames: Cassy (Most), Rory (Sam and Dean), Bug, Little Bug, Mini Millan (Dean), Flower, Petal (Crowly) Abomination, Half Breed (Angels)
Species: Nephilim
Alias: Angel, Guardian
Age: 16
Face Claim: Marie Avgeropoulos

Occupation:
Hunter
S.H.I.E.L.D agent (briefly)
Avenger (Future- short term)
Pool Hustler
Man (Woman) of Letters (Legacy)

Family:
Adam (Ancestor) †
Eve (Ancestor) †
Cain (Ancestor) †
Abel (Ancestor) †
Henry Winchester (Paternal Great-Grandfather) †
Millie Winchester (Paternal Great-Grandmother) †
John Winchester (Grandfather) †
Dean Winchester (Uncle)
Sam Winchester (Father)
Adam Milligan (Uncle)
Emma (Cousin) †
Bobby Singer (Surrogate father)
Archangel Ariel (Mother) †

Background:

When Sam was 16 he met a woman named Ariel. They fell in love, and the whole relationship was hidden from John and Dean. Sam had never met a woman so beautiful in his life and couldn't imagine ever losing her.

But of course, nothing lasts forever when you are a Winchester. Eventually they had to move to the next town in search of a case. The very day they had to leave Ariel had come to him, frantic. She was trying to tell him something, and she looked terrified. Sam tried to listen, but John had pulled him away from Ariel. Sam had told her to try and find them.

Nine months later Ariel had shown up at their hotel room in Memphis, Texas. Sam called John who rushed over to find Sam holding a baby in his arms. There was a lot of arguing, and at some point during this Ariel had fled the hotel room. John agreed to keep the child so long as no one knew it was Sam's.

Sam quickly realized Ariel had disappeared and went out to search for her. Eventually his search led him to an abandoned farm outside the city limits. There he found Ariel lying on the ground with a hole burned into her chest. When he returned to the hotel room, John had told Dean that the baby was his sister. Sam tried to tell John that something had killed Ariel, but he said he did not care. This is another thing that led to Sam leaving.

Little did Sam know that the woman he fell in love with was an archangel. Ariel's brothers and sisters were sent to find and kill her and the child, but only found Ariel. They tried to find the child or the father, but Ariel had carved Enochian angel warding into Cassandra's ribs. They never found her.

Cassandra never had a time in her life that she didn't know about the supernatural world. John had drilled her head with lore and constantly trained her. He never treated her as a person, but more of a burden. Sam and Dean did whatever they could to make her life good. Cassy would often bring animals home as 'pets'. She once brought a baby mountain lion back to the hotel.(Sam and Dean were terrified) Birds would land on her hand often and she would easily calm animals down. Dean would call her Mini Millan after Cesar Millan, the dog trainer. Sam was often cold toward her because she just reminded him of Ariel.

"Sammy, play with me!"
"Not right now Rory."
"But you never play!"
"I said not now Cassandra!!"

Dean would often get confused by the way Sam and John would treat Cassy and would fight with Sam about it alot.

When Cassy was 10 there was a massive fight between her 'brothers' and 'father.' Sam wanted to leave to go to college and live a normal life. He considered taking Cassy with him, but decided against it. At one point Dean leaves to go get something and the fight between Sam and John carries on. They had thought that Cassy had left with Dean, but she had hid in the closet.

During this fight Cassy finds out that Sam is actually her father. Her whole world comes to a halt. The fighting started to intensify and Cassy kept chanting in her head I want to leave, I want to leave, I want to leave. With a gust of wind Cassy opened her eyes and found herself in a field of flowers under a full sky of stars. This was the first time Cassy had used her powers.

Several animals such as squirrels, birds, and even larger animals like wolves started to surround her and comfort her. She stayed there for hours, happy in her own world of no fighting until Dean found her. The animals ran away before he could see them and Dean took Cassy back to the hotel room. Sam was gone, and Cassy never confronted John about the fact that he was her grandfather.

Over the two years that Sam was at college Cassy had found that she had 'abilities'. She would sometimes teleport in her sleep and found that she could understand what animals were feeling. Sometimes even have full conversations with them too.

When John went missing, Dean had told Cassy that they needed Sam. She didn't speak the whole drive to Sam and this is when Dean realized something had happened between Sam and Cassy. He never asked though.

The night that Jess died Cassy had a dream of younger Sam and another woman with brown hair who looked very similar to Cassy. They seemed madly in love, but the dream had shifted to Sam cradling a baby and then him holding the woman. He was crying, and the perspective of the dream raised to where Cassy could see the outline of wings burned into the ground around the woman.

Cassy woke up and knew that, that woman was actually her mother. She'd realized that her abilities came from her mother. She tried to find more about her, but after a lot of looking, she found that the woman had never really existed. Cassy started to have doubt about her dream, but something changed her mind.

Another dream. The woman had appeared to Cassy again, and had explained what and who she was. Cassy asked to meet her, but Ariel told her she was dead.

Once Cassy woke up she delved her personal time into looking at lore about Nephilim's and found that her mother was actually the Archangel Ariel, the angel of Nature and Animals.

Pieces of Cassy's life started to make sense, but came to a halt when she and her family got into a car crash. Cassy had been greatly injured to the point where she should have died instantly, but in the time that it took the paramedics to arrive, Cassy didn't have a mark on her.

At the hospital Cassy could see Dean walking around outside his body. She helped him because she could see the Reaper too. Dean tried to question her about it, but she didn't say a word.

When John summoned the demon she could feel its presence, but couldn't find it. Eventually Dean woke up and didn't have any memory of what had happened during his out of body experience. Cassy was extremely relieved. When John returned, Cassy knew what he had done, though she didn't know how she did. When John spoke to Dean he had also told Dean that Cassy wasn't fully human and that if she was to do anything that he would have to kill her. Dean never told Sam or Cassy about that because he didn't want to admit that Cassy wasn't human.

When Sam dies Cassy tells Dean "I can't exist if either of you don't. I have to do something about this." Dean tried to stop her. They fought for a bit and Dean eventually had Cassy in a pin to the wall. Her eyes started to glow a mint green color. She yelled at Dean to let her go and in the process of this she flung Dean off of her and into a brick wall. Dean was knocked out cold, and Cassy got into the impala and drove until she reached the cross road and summoned the demon.

"I don't care how long you give me, just bring Sam back."

The demon gave her one year. When she returned she found Sam and Dean up. Dean had known what she did and kept giving her looks, but she never met his eyes. She showed no remorse for what she did.

When they arrived at Bobby's house Bobby was trying to decipher which one of the Winchester's had done the deed, but Dean had made eye contact with Bobby and looked at Cassy who had her eyes to the ground. Bobby had Sam check the map and pulled Dean and Cassy out to the yard.

"You dumbass Rory! How long did they give you?"
"What's it matter?! Huh? Sam is alive!"
"Cassandra! How long?!"
"One year."

Dean tried to ask about how she knocked him off of her, but Cassy deflected it by saying they needed to find the yellow-eyed demon. Later they left for the cemetery and surrounded Jake. When Jake told Sam that he killed him all eyes landed on Cassy and she didn't take her eyes off Jake, her jaw set and ready to kill. Jake tried to have Cassy point her gun at Ellen, but she seemed unaffected by the mind control, so he made Ellen put the gun to her own head. Dean eyed Cassy and started to realize what John said about her to be true. She wasn't fully human.

When Jake turned to open the gate Cassy's eyes began to glow again and Ellen was able to take the gun away from her head. She looked at Cassy and saw her eyes. Sam shot Jake then went in front of him and shot him several times in the face. The gate began to open, Cassy grabbed the colt, and everyone ran for cover. The gate to hell bursts open. Cassy sensed the demon behind her and aimed the gun at his face, but it took it from her before she could fire. He sends her flying and she hits her head on a headstone. Sam saw her lying on the ground and ran to her aim, only to be pinned to a tree.

Azazel was about to kill Cassy when John's spirit grabbed him from behind. Cassy took this moment to grab the colt and when Azazel stood back up, she fired. Sam falls to the ground, and Dean, Ellen and Bobby shut the gate to hell.

John looks at Cassy and she begins to cry. She felt how much he loved her in this moment and how much he cared for her. Cassy instantly forgave her father.

John gives his sons a look of how proud he is, then steps back and disappears in a flash of bright light.

Sam, Dean and Cassy all stood in front of the impala, when they both decided to confront Cassy. "Rory, you saw Jake's face when he saw me. He said he killed me. What happened after I was stabbed." Dean looks at Cassy, arms crossed. "Yes, please tell us. What did you do after Sam was stabbed?" Cassy bites her bottom lip and looks off into the woods. "Rory. What happened?" Cassy doesn't answer, and Dean shakes his head. He gives Sam a look of confirmation. "You sold your soul; Didn't you?" Sam stands in front of Cassy. She sighs and looks him in the eyes. Her eyes were brimmed with tears. "Yes. I did. And I'd do it again." Sam's eyes tear up and his voice cracks. "How long did you get?" Cassy tells him, and he yells at her saying she shouldn't have done that. She once again says that she doesn't regret her decision. "Rory, back at the house, something happened." Cassy glances at Dean and can feel Sam's eyes boring into her. "Care to explain what that was?" Cassy scoffs and wipes away her tears. "No."

As Cassy's year goes by, all their efforts to get her out of the deal were for not. Eventually came the day where she would go to hell. Lilith came and set her hellhounds on Cassy. Sam's heart broke watching his daughter being torn to shreds. He never got to tell her how much he loved her. Just like Ariel...

Four months, or in Cassy's case, 40 years go by and she is resurrected by Castiel. When bringing her back he found out she was Ariel's daughter, but kept his mouth shut. He loved his sister and hated it when the order was set out to have her killed. He refused to help, and now to honor his sister he never told anyone about Cassy.

When they found out angels existed, Cassy wasn't shocked at all.

"I've known about angels for awhile."
"Why didn't you say anything?"
"Would you have believed me?"
"No."
"My point."

Castiel tried to avoid her as much as possible so he could best keep the secret, so he chose to speak to Dean on all matters.

So her backstory was going to continue on but I got tired of basically rewriting season 4. Same stuff happens basically. Sam and Dean still don't know what she is but they are suspicious. Coulson gave Rory her job offer just before the mid part of the season. This takes place in season 5

Personality:

Cassy is a very complicated person. Her emotions are all over the place. When she was younger she was a very fun and open child, but as she grew up into the world of hunters she started to shut down. Deep down she always knew she was different, and it played a key part into why she never told anyone she is a nephilim.

"Rory, wasn't the same after she got out of the pit. She shut down and let no one in."

While in hell Cassy went through more tourment than anyone could ever imagine. Alastair knew what she was and wanted to make her suffer. Cassy realized what being a nephilim meant and how other creatures felt about her. When she found out who her father really was, she decided to never trust anyone ever again. This really hurt her but you'll never hear her say that.

Animals are drawn to Cassy and she sometimes really dislikes this trait (only when people see it). Despite being cold toward people, Cassy finds it easy to be herself around animals and nature.

Trivia:

- Sam is aware that Cassandra is his daughter, but due to John (See Backstory) she was told  that Sam and Dean were her brothers. She eventually found out, but never told Sam or Dean. Dean has no idea that she isn't his sister.

- Her brothers are very protective of her, even though she could possibly be more powerful than Castiel.

- Castiel hated Cassandra at first, since she was an abomination, but when he started to get close to her brothers, he grew fond of her till the point where they act like siblings.

- Sam and Dean don't know Cassy is a Nephilim. She, Cas, and other supernatural creatures do. They are aware that she isn't fully human, but everytime they ask her she never answers. She's kept it secret because she feared how her family would react, because she learned in hell how much Half-breeds were treated.

- Sam is afraid of clowns, and Dean is scared of planes, and Cassandra is terrified of the dark. This mostly stems from her time in hell.

- Just like her uncle, Cassandra LOVES classic rock. Her favorite is Queen, with AC/DC not too far behind. Sam is okay with this so long as she listens to 'modern' music occasionally. (She also likes Imagine Dragons, and Billie Eilish.)

- Cassandra is in simple terms; a badass. Do NOT get on her bad side.

- Just like her Dad and Uncle do, she says things in sync with them

- In rock paper scissors she uses gun. Sam and Dean let her get away with it because she is younger.

- Cassandra can often be found twirling her throwing knife. It's like her very deadly fidget. People feel intimidated seeing a 16 year old twirling a deadly weapon casually.

- Cassandra doesn't need her powers to kick ass. In fact, she hardly replies on them, mostly to keep them hidden from Sam and Dean

- Being a Nephilim, Cassandra is impervious to demonic and angelic possession

- She is warmer than the average person. This is a trait from being a Nephilim.

Pairing: Peter Parker

Abilities:

Master Hunting skill: Trained by her Sam, Dean and John from a young age, Cassandra poses excellent combat and hunting abilities; she is more than capable of taking a stand against even the most formidable of opponents, such as demons, vampires, and werewolves

Marksmanship: Cassandra is well acquainted with many types of firearms, and is an expert marksman. Her skill rivals her Uncle, Dean.

Master Swordsman: Cassandra is very skilled with knives and bladed objects.

Multilingual: Cassandra is fluent in English, Latin, German, Russain, Enochian, and a few others.

Equipment:

1911 colt handgun
Throwing knife
Pocket knife
Sawed off shotgun
Demon knife
Angel blade
Archangel Blade

(I have yet to write out her powers but they are basically the same as Jack's, except she has more influence with animals and nature.)

Bonus!

Quotes:

"You just stabbed me in the boob. Oh, you are SO dead."

"Your parents let you get a tattoo?!"
"My parents are dead."

"What are you?"
"I'm a Winchester motherfucker."

"Go to hell!"
"Oh honey, where do you think I came from?"

"You just broke my nose!"
"Ask me if I give a shit."

"Jeez, remind me not to get on your bad side."
"You haven't even seen my bad side yet."

"You want a fight? I'll bring on a war."

"She's proof you can walk through hell, and still be an angel."

"You're the devil!"
"The devils got nothing on me, my friend."