Castiel is Hebrew for 'Shield of God'. I found it by accident while looking through Wikipedia recently...and a plot bunny was born.

It was all over. Lucifer had won, all because Heaven wanted to end the war between the two arch angels and damn the consequences. Through it all, he had tried to keep his faith in humanity right until the end.

But Castiel had lost faith in himself a long time ago. Turning to drugs didn't work, but it dulled the pain of his wings. Turning to alcohol was useless, but it stopped him from remembering the pain of being banned from Heaven by Micheal. And carnal pleasure hadn't interested him since he learned Dean no longer wanted anything to do with him.

He had tried. And for so long he had held out hope that maybe things would work out. That maybe Dean would learn from his mistakes...instead he became his father and shut everyone who tried to help him out.

So Castiel waited. Waited for the end of everything so he wouldn't have to feel anymore.

And when Lucifer came to the camp, wearing Sam's face, his heart wept for humanity. How could this have all ended? Why did Micheal insist on forcing the battle between the brothers and start the end of the very creations their Father had loved most of all?

Castiel was to be the last to die, an example for what Lucifer would bring upon the heavenly plains all because of Micheal's arrogance.

He found himself chained beside the Prophet, who had ceased to get anything. All he saw was darkness now.

Chuck coughed.

"After all that they went through, it ends like this?" he said.

"It could be worse."

"How?" asked Chuck. How could anything be worse than this?

"I could be sent to hell instead of being condemned to nonexistence."

"How is that better?" asked Chuck baffled.

"I said it could be worse. I never said it couldn't be worse for you. At least I won't have to deal with this anymore," said Castiel bitterly.

"How is nonexistence better than hell?"

"Because in the event I die, I get erased permanently. If that doesn't happen I'll be stuck for eternity on Earth after Lucifer's done with it and forced to see my failures every day until the end of time," said Castiel flatly.

"Hey Cas... if you could change things back when there was still hope, would you?"

"You mean stop the angels from succeeding in letting him out? I haven't got that kind of power, and even if I did still have the ability it wouldn't work. The same soul can't exist in the same time twice for more than half an hour at most. There's a reason why time-travel is restricted without the approval of a higher angel."

"If you could go back and changes things, would you?"

Castiel looked around him. This wasn't what he had fought for as a soldier and it was all Micheal's fault for starting this war. A war that the angels lost. He had only made a temporary solution when he sealed off heaven...Lucifer would eventually break in and lay waste to all of creation.

So the idea of changing things? Righting the wrongs committed by his brothers? If he had the power and could get away with it, he would have done it years ago. Long before Zachariah dumped Dean's past self in this timeline in fact.

"If I had the ability to stop this... I would do it without hesitation. This wasn't what our Father intended when he created us. He never wanted the angels to start the very thing they should have prevented by dealing with the demons," said Castiel finally. Chuck was soon to die. He was next in line for the racks.

"Do you still believe in your creator?"

"After all the pain I've gone through... I still believe. It's heaven I have doubts in," admitted Castiel.

"That's all I needed to hear," said Chuck, sounding much stronger than he should have.

Castiel blinked and turned. Time had frozen all around them. Lucifer was paused in mid gloat. He looked, and really looked at Chuck.

"Who are you?"

"I'm sorry I couldn't stop this. Because humanity always had the chance to end this war before it started, my hands were tied. Free will is the gift to all creatures, and it was free will that forced my hand. But the others have gone too far, and I'm not about to let the end of all things be caused by my own sons. I can give you three gifts, Castiel. But it's up to you how you use them."

"Father?" whispered Castiel. He had been so close to his creator and never known. He felt like a fool.

"Do you truly wish to stop your brothers from ruining existence? I can only give you one chance, but I refuse to let humanity end because of arrogance."

Castiel felt the tears fall from his face. He had long since given up being an angel, he was more human than that.

"I don't want this to end because of heaven. I always had faith that humanity should have the chance to be safe from this. Why, why couldn't you have told me about this yourself?" said Castiel, his voice breaking as he almost shouted.

"I told you. Free will tied my hand. There is one chance to make things right, but only if you're willing to fight for it."

"What do I have to do?"

"Stop the others from tearing the brothers apart."

"Any other impossible tasks you want completed?" asked Castiel bitterly.

God held Castiel's chin in his hand.

"I brought you back once for a reason. Because I believed you might be able to save the Winchesters from themselves and those that sought to use their bloodline to end the world. But you're the only one left, Castiel. You have to make this choice on your own."

"Free will huh? If all it took was free will to rebel against heaven and save them, then I will live up to that hope. Just don't expect me to be a perfect little soldier anymore," said Castiel bitterly.

"All I ask is that you live up to your name. Castiel, the Shield of God."

Castiel laughed bitterly. If he was a shield, then he must be dented and broken almost beyond repair.

"Remember Castiel this is the only chance I can give you. I refuse to let Lucifer's hissy fit end everything."

God put a hand on Castiel's shoulder, and he felt something inside of him change. He was openly sobbing when he felt his Grace return and his wings healed. He was so relieved to be whole again that he failed to notice the other changes his Father was putting him through.

When he was done, God snapped his fingers in the silence around them.

Castiel felt himself falling through time and space...before he blacked out completely.

The first thing Castiel noticed was the voices. Just being able to hear them at all would have made any angel weep after what he had just left. Opening his eyes, he noticed his center of gravity had shifted dramatically.

He was in the same clothing he had worn almost constantly when humanity still had a chance. Getting up, he decided to see where and when he was.

That is until he passed by a store with a window that showed his body to him, and did a ten second standstill.

He wasn't a he anymore. The body that Castiel inhabited now was most definitely female in every way, shape and form. He...she had breasts for crying out loud. (Further inspection in a bathroom revealed she had all the equipment, not that she had ever gotten the point of caring for that part to begin with. Angels were asexual originally.)

This...presented a small issue. Castiel was no stranger to the way women were treated differently, despite the fact that one of his commanders had a preference for the female forms.

Women were generally weaker in the muscle mass compared to the males. The also tended to be more hormonal during times of the month, and had the ability conceive life despite all odds.

Castiel looked at herself from a small cafe after stealing someone else's wallet to pay for her drink. Living as a human without any hope had adjusted her morals. She had no issues with theft. Especially from some idiot stock broker who had a contract.

Realizing that there was no real way she could fight without knowing the limits of her body, she decided to find a place to learn hand-to-hand so she could get used to this new form.

Castielle (she had decided to change her name slightly to avoid picking up on any prayers or orders directed to her) walked into the college tiredly.

She had been picking up hints of Grace heading towards this area for over a week, and despite having finally gotten used to her new body (and picking up a new fighting style in the process) she was not looking forward to this. At all.

She had already figured out when she was... it was about a year after Dean brought Sam back from his only successful attempt at being a civilian. Which meant she had time. Time she wasn't going to squander.

That also meant she needed information, and who better than the very bastards who started this entire mess to begin with? So she went angel hunting.

Whoever she was looking for was good. Clearly they had been on Earth for quite some time to figure out how to hide their presence, or disguised as something else. But they were careless. They couldn't hide all the signs, and whoever this was was high up enough on the angelic food chain that she might get some decent information. Enough to possibly work with.

The problem was that she had to wait. Fortunately there was a library here, and so long as she never checked anything out they wouldn't know she didn't belong.

A few weeks in, and she was finally up-to-date with popular culture enough to make sense of some of the things Dean had always talked about before the day he gave up on Sam. Though she still had plenty to research.

She closed her current book, felt the crick in her back, and decided to head to the bar.

About halfway there she caught the sensation of angel Grace, almost hidden but still there. She looked around until she spotted a man who hadn't been on campus the day before pretending to be the janitor.

Clever. No one looked twice at the people who cleaned the place. Now she just had to corner him in an area that the cops wouldn't be called into.

Castielle waited until night, when the school was practically emptied out. She hid her presence until the man was alone. Then she slammed him into the stone wall with perverse glee.

"OOF! What the hell?" he complained. He noticed the breasts on his back and then quipped, "If this is your idea of showing interest sweetheart, you ain't gonna get many boyfriends. Unless they're masochists."

Castielle's eyes narrowed and she made sure that the blade in her hand was firmly at the base of the spine. Judging by the rising power level from this jerk, she had caught someone very important indeed. Possibly a seraphim.

"Shut up. Who's your garrison commander, seraph?" she growled.

She sensed his alarm at being caught out for what he really was, but his chuckle caught her off guard.

"Wrong choir sweetheart," he quipped.

His power was too strong for anything lower than a seraph... her eyes widened. There was only one angel that powerful who had gone missing.

"Gabriel."

If he had been surprised before, he was floored that she had figured it out so fast.

"Got it in one. So what are you and how in heaven did you get your hands on an angel's blade?" he asked. It was hard not to recognize that power on his back.

She let him up and he finally got a good look at her face. He didn't recognize her offhand, but he recognized the blade.

"No way. You possessed a girl?" he said in open amusement.

"Shut the hell up coward," she growled, "I didn't chose this form. It was altered so I could exist twice."

"What did you call me brat?" said Gabriel pissed. He wasn't letting some no-name foot soldier get away with that brazen attitude.

Castielle glared at him full force, and something in her eyes made him flinch. This wasn't one of Micheal's garrison soldiers anymore...this was someone who didn't have anything left to lose.

"Let's get one thing straight coward. I don't give a damn about the fact you're in hiding from above and below. I don't care if you draw every hunter within fifty miles pissing people off. I'm just here to insure that the planet doesn't get destroyed because heaven just had to have the fight between the morons who spew bullshit with every word," said Castielle bitterly.

Gabriel stared. Since when do foot soldiers openly insult Micheal?!

"And lets not forget the idiot who had the world's biggest hissy fit all because Father loved the humans more than us. We had one simple fucking order, to love and protect humanity...and the idiots can't even do that right!"

"Who are you? I've never heard an angel openly insult Mikey, let alone speak against heaven like that."

"My name is Castielle, Messenger, and our Father sent me back to insure Lucifer doesn't win twice."

Gabriel all but crashed on his couch that night.

He had been hoping to avoid dealing with heaven since he left, but being confronted with the cold hard fact that if he didn't do something Lucifer would not only get free, but win had been a slap in the face.

Castielle (and he had been slightly amused that the mindless foot soldier had fully embraced humanity and his new feminine form enough to take on an altered version of her true name) had been blunt and entirely too angry over the fact Micheal had ruined everything all because he wanted to get the fight between him and Lucifer out of the way.

Though Gabriel had no idea why Cassie was still hanging around after she found out who he was and how he had been hiding. He had already given her his cell phone number, so what was keeping her here?

He got his answer when he saw who had come to hunt the 'Trickster' persona he had adopted.

The Winchesters.

Castielle admitted her primary mission was to keep heaven from splitting the brothers apart.

According to her, everything went to hell rather fast when Dean had enough of Sam drinking demon's blood despite the fact it was Zachariah's fault for letting him out to go seek Ruby.

However that confusion cleared up rather quickly when he went to the bar to observe the idiot duo.

Seeing how Castielle focused almost entirely on Dean, it didn't take a scientist to figure out that the angel had a deep crush on the older Winchester. One that hadn't gone anywhere from the look of pain on her face.

No wonder she had been picked.