They were each other's everything.

If she tried, Gamora could remember a time before her sister was there. The memories were vague, more like dreams than something concrete and real, but they were there. They were buried somewhere in the back of her mind. If she wanted, she could remember what it had been like to be alone.

Most of the time, she didn't want to remember.

"This is Nebula. She will be your sister." A pause, almost like a warning. "She is mine, just as you are."

Gamora eyed the girl standing in front of her suspiciously. She looked to be similar in age to herself, still a child but not an innocent. Her eyes made that clear.

Thanos only wanted the best underneath him. He had no use for weakness. This Nebula must have proved herself to him.

That did not mean she had proved herself to Gamora.

"Hello, sister," Gamora said with a nod of her head.

Nebula nodded back. Her eyes were just as suspicious. "Hello, sister."

Training was harsh, but neither complained. They learned. They followed orders. Once it was finished, they silently helped each other clean themselves and take care of any injuries.

There were usually injuries.

"You depend too much on yourself," Nebula said out of nowhere, breaking the unspoken rule that they had created.

Thanos may have made them sisters, but he did not make them friends. Gamora did not trust easily, and she suspected Nebula trusted even less.

Gamora looked up from the gash on her leg that she had been closing. "Who else should I depend on?"

Nebula met her gaze. "We're both here to serve, are we not?"

Gamora's weaknesses were balanced by Nebula's strengths, and Nebula's by Gamora's. They were a good match. Thanos must have seen that when he took her sister and made her his own, just as he had with Gamora all those years ago.

A part of Gamora, buried deep within, wondered if that was why he had taken Nebula in the first place. If her sister had lost everything, just as she had.

She never asked. They still did not trust one another, not outside their training, but it did not matter.

As Nebula had pointed out, they were both there to serve.

Training finished. Work began.

Killing was an art. They were still apprentices, honing their trade, but they had learned enough to be more than mere amateurs. With every death, they learned something new. In the field, they worked together to do whatever their father asked of them.

He had many enemies. Thanks to them, he now had fewer.

Their father smiled at them, a cold expression that sent a shiver down Gamora's spine. "I wish to see which of you is the strongest."

Neither of them hesitated. They had learned that much, over the years.

Gamora threw herself at Nebula. Nebula threw herself at Gamora. Kill or be killed, that was the rule. Don't care. Don't feel. Don't trust.

It wasn't quite as easy as it should have been, to fight her sister and not hold back. To try to kill her. Nebula felt the same way. Gamora saw it in her eyes, just for an instant, as she straddled her and held a knife to her neck. Then her sister managed to flip her and hold her in the same position.

They were evenly matched. They always had been. Gamora's weaknesses were her sister's strengths. Nebula's weaknesses were her own strengths.

Thanos tired of watching them before either had managed to gain the upper hand.

Gamora saw the anger and frustration shining in Nebula's eyes as they watched him walk away after dismissing them. She suspected it was a mirror of her own.

Neither of them said a word.

That night, as they bathed, Nebula leaned in and kissed her.

After a few seconds, Gamora pushed her away. But not before she kissed back.

Gamora's eyes were wide as she pressed down on what was left of Nebula's arm. They had had missions go wrong before, but never like this. Their previous targets had always hesitated, if they hadn't died outright, an extra millisecond where her and her sister's youth had protected them. They had learned to depend on it, over the years. Their enemies had seen children standing before them, not assassins, and that mistake had always been enough to see their job done.

This time, there had been no hesitation. No extra moment.

She and her sister were turning into women, and they hadn't even noticed. And now Nebula was paying the price.

Are you scared? she wanted to ask.

I'll keep you safe, she wanted to say.

Gamora leaned in and pressed a harsh, angry kiss against her sister's lips. "Don't die," she said, pulling away.

Nebula nodded. Her sister knew an order when she heard one.

Lips on lips. Skin on skin. Teeth on teeth.

Sex was just another battle for them. A struggle to gain the upper hand, both of them perfectly matched. A fight for dominance, to see which one of them would end up on top.

Neither of them ever won. Or, perhaps, both of them always did.

It was one of the few things that was theirs, just theirs. It was the one time they could pretend, for just a short time, that Thanos was not holding them and pulling their strings from the darkness.

It was an illusion, of course. Both of them realized that much.

Gamora pulled away from Thanos. Not on the outside. She wasn't a fool. But inside? Inside, she began to think. She hated him for everything he had done to her. She wanted to see him fall.

She listened, and she waited. The time would come eventually, of that she was certain. The universe would give her a chance, and she would take it.

Part of her wanted to tell her sister. She didn't.

Or, more accurately, she didn't have to. Nebula knew.

Gamora could see it in her sister's eyes when they fucked each other against a wall in the aftermath of an assassination. She could feel it in her sister's hands as they pushed her closer and closer to release. She could hear it in her sister's groans as Nebula was pushed over the precipice herself. She could taste it in her sister's mouth as their tongues continued to fight the battle that their bodies had called as a draw.

Yes, she was well aware that Nebula knew. What she didn't know is where her sister would stand when that moment finally came.

Gamora missed her sister. They were each other's everything. Two parts of a whole, for almost as long as she could remember.

Sometimes she wondered if her sister missed her.

(Nebula did.)