Let's assume Jyn Erso survived Rogue One. If she knew that Luke (because it was probably Luke) planned to leave Rey alone on Jakku, there is no way she'd allow it.
Luke raced to the Jedi temple. He never should have left, even briefly, but he had been confident that, if necessary, the Jedi trainees could hold off any attack, at least until he got back. He had never anticipated an attack from within.
When he returned, it was too late. Smoke and bodies everywhere. But he knew that. He had felt each one's terror and then their silence in the Force. Still, he physically checked each student. To his surprise, one was alive. It was Rey, her tiny form curled into a ball. Her breathing was shallow but her pulse was strong. Reaching out with the Force, he could feel her emotions - anguish and horror. Strange that he had not noticed it before. He supposed that in his own grief, he hadn't been able to distinguish hers.
There were no physical marks on her; no blood. Nevertheless, it was clear she needed medical attention. Her raspy breathing indicated smoke inhalation - or attempted strangulation via the Force.
He already knew that the medical room of the Jedi Temple was unsalvageable. The bodies of the trainees who had run the center had been barely recognizable. He shuddered. He had to get her off planet, and quickly.
He placed Rey in a healing trance and gently carried her to his ship. For all its clunkiness compared to the fighters he used to fly, at least it had room for passengers.
Luke began running through a mental list of possible santuaries. Since he was looking to lay low, that ruled out most of the nearby systems. Then he remembered some friends from his days in the Rebellion had an outpost nearby. After helping topple the Empire, they had pooled their pensions, bought a couple frieghters and started a shipping company, of all things. It was all very above-board, or so Luke had told Leia after a visit a couple of years back. In reality, they shipped more medical supplies than they reported and sold the surplus, under the table, to those who couldn't afford it otherwise.
The company was quite successful and they had several bases of operations across the galaxy. One was just a short jump away. Luke set the destination, ran the calculations and hoped someone he knew was on this particular base.
After an uncomfortable conversation with the traffic control tower - eventually involving a Jedi mind trick - he heard a familiar voice over the comm. "Erso speaking. How may I-"
"Jyn, thank the Force! I've got a wounded student. Do you have a droid that could take a look at her?"
"Luke?"
"Yes."
"Of course. Land in, oh, Bay 13c. What happened?"
"I'll tell you later. And if anyone asks, my name is, um... Owen Lars."
"Understood. I'll meet you in the clinic. My people at the landing pad will help you get your student there."
There were two men with a stretcher waiting for him when he landed. They led him to a medical center, larger than Luke would have guessed a facility of this size would contain. Of course, that was the last thing on his mind.
Jyn helped move Rey to a bed and asked Luke for a summary of her injuries. The droids began their work on the girl. Jyn asked, "Luke, what happened?" She listened sympathetically as he clinically recounted the events. Somehow it felt less real now that he was explaining what happened. His head was spinning.
"You're pale. Sit down," Jyn commanded. She called for some broth. "What else do you need?"
"I'm - we're," he corrected, nodding his head in the direction of his charge, "going to lay low for awhile. As soon as she's healed enough to leave on her own, we'll head out. I don't want to put you in any danger."
"Take as long as you need. Do you need suppli- Wait. What do you mean 'leave on her own'?"
"I can't take her with me. I'll go to I don't know Jakku, maybe. I'll need to wipe her memories, block her connection to the Force -"
"Stop. Use your head. That's a terrible idea."
"No. I can't take her with me, I -"
"Bantha shit! You are not leaving that innocent, broken girl by herself on some backwater junkyard."
"But she won't be safe-"
"I'll take her in. She'll be safe with me."
"I can't let you-"
"You listen to me, Luke Skywalker, she is staying with me, or do you want me to hack off your other arm?" Somehow, through the course of the conversation, Jyn had managed to steal Luke's lightsaber. She now had it pointed at him, un-lit but threatening nonetheless.
"You'll do whatever it takes?"
"Change her name, alter her appearance, go underground with her at a moment's notice if I have to. My friends and I know a thing or two about hiding."
"Th-Thank you," he said, visibly releaved.
"Luke-"
"Yes?"
"What's her name?"
