My beta is Friglit. Characters belong to J.K. Rowling.

Could be Ron.

Fleur was wonderful even with an apron and a ponytail. Bill, who was supposed to help her preparing the dinner, had been distracted by her concentrated expression and was looking at his wife with admiration and love, especially after she had opened the window and the wind had started swinging her beautiful silver hair.
Absorbed as he was, the sound of an Apparation caught him unprepared, but noticing his wife's alarmed expression he instantly grabbed the wand and motioned her to move away from the window: there was the Fidelius Charm cast on Shell Cottage, so the intruder had to be someone of the family, but it was better to keep "constant vigilance", especially since no Weasley had informed him of a visit that day.
Bill moved to the door and gave a quick look outside spotting a figure standing in front of the house, with bright red hair and ragged clothes.
The wand almost fell from his hand when he realized who the stranger was, and Bill ran out just in time to see Ron collapse to a kneeling position, as if someone had cut his legs. Bill felt himself panic at the thought that his little brother was wounded, perhaps seriously, but rushing beside him he quickly saw that there was no sign of blood, just a few scratches that didn't seem deep. Ron's clothes and hair however were still soaked and the poor boy was clearly devastated.
"Bill!" cried Fleur, catching up, "Ron?"
"What happened?" Bill asked, feeling like his heart was in his throat. "Who..." but he couldn't continue, it was impossible to ask if Harry, their hero, was dead, or if it was Hermione, the little bookworm that had grown alongside his brother. It was unthinkable. "Are you okay?"
"No," answered Ron hoarsely. Bill realized that he was almost crying, all his body was trembling violently.
"Come here," he then said gently, helping him to stand up and pushing him toward home, "Fleur, please, could you get me some dry clothes and blankets? Maybe better light a fire too..."
Ron didn't talk that night, limiting himself to a dry "They're fine" before fixing his gaze on the flames crackled in the fireplace. He didn't touch any food, despite the fact that Bill's clothes hung off him where just four months ago they were too small, and despite the ghastly pallor of his face, where now freckles now stood out like wounds.
The next afternoon Ron sat next to him and told him everything: that he had travelled with the other two and that he had begun to misbehave - influenced by something that he couldn't explain, and finally all the resentment had resulted in a final fight with Harry.
"And I left them. I tried to go back but they're protected by spells to hide their presence… I even met some snatchers... Long story, don't ask. I've lost them," he ended hopelessly, with his shining eyes fixed on the sleeve of Bill's shirt, too ashamed to look him in the face.
"But... After all you've done, after everything you said… How could you abandon everything just because of a fight?" asked Bill, disappointed in his brother. He had expected something more from him after the way that Ron had prepared all summer for this trip, not flinching even though he faced the most dangerous journey ever faced in living memory. Ron had managed to worry even Fred and George, to be honest, and that was a first.
"I don't know," whispered Ron, finally meeting his eyes for a moment.
Bill froze, because his brother was indeed destroyed, there, in front of him, with his heart in his hands and ready to accept the reproach that he felt he deserved.
Bill couldn't, couldn't add to the weight he was carrying.
"I failed," continued Ron bitterly. "As always."
"You have never failed, Ron" Bill corrected him, placing his hand on his shoulder. "Now I want you stay here for Christmas and rest. After that, if you want, go back to look for them, but not in these conditions. "
"They would never... They would never want to see me again." whispered Ron.
"Don't talk nonsense; I'm sure they can't wait to see you," he replied, hoping to sound more confident than he felt.
"What will you not tell mum?" Ron then asked, "Please don't say that I left! You know what would happen! "
"I'll invent some excuse, don't worry," Bill reassured him immediately, "But now you need to eat something. Fleur will hurt us otherwise."
So Ron stayed for Christmas, constantly depressed. The only thing that made him smile at all was when Bill told him that Lupin had returned to Tonks; after that there was darkness again.
Then, one morning, Bill awoke with a strange feeling. As he went down into the kitchen and saw a note on the table: he knew that Ron was gone again.

One spring night, months later, he again heard the powerful sound of an Apparation. But this time was more than one and Bill raised again the wand, turning the light outside. He knew that the Incanto Fidelius was still working, Kinsgley came to see them that evening.
"HELP!" he heard yelled in a desperate tone, and Fleur, hugging her dressing gown around her, appeared beside him with her wand in her hand. A moment later he saw four figures, three tall and one small enough to be identified as a house-elf.
"Who…" he asked, getting closer.
"Dobby must go to save Harry Potter, sir! The Wheezy sent me here with them! Dobby must go!" announced the elf, looking scared to death before Disapparating again.
The boy and the girl, because they were clearly only two students, hugged each other in order to keep standing, and a third staggered towards them.
"We're Dean Thomas and Luna Lovegood. You're Bill, right? The others are locked at Malfoy Manor, they saved us... The Death Eaters kept us trapped in some basement..." explained the boy, panting, while the blonde girl looked at him with wide eyes. Bill winced, remembering that she was Ginny's friend, the daughter of Xenophilius Lovegood, kidnapped on the train back from Hogwarts.
"Ollivander?" exclaimed Bill, recognizing the man behind the children. He looked back at Dean, "Wait, do you said you were trapped at Malfoy Manor?" he asked, thinking of sending a Patronus to the Order immediately.
But in that moment he heard the noise of a second Apparation, this time closer to the hill in front of the house.
"Must be them," said Luna happily. Bill glanced at her, hoping she was right.
A moment later the silence broken again by screams: "HELP! HELP! "
"Harry!" Dean groaned.
Bill jumped into a run, followed by the others.
"BILL!"
Ron's voice reached him like a breath of fresh air, but the relief lasted only a moment. His wand was creating only a feeble light, but he still managed to make out the blood. Bill cast the light over his brother and saw that he was holding Hermione in his arms: she was unconscious or perhaps even dead, and he seemed about to faint.
"What… How... "
"I need to get Hermione in your house," declared Ron, faltering dangerously. Bill grabbed his arm.
"Let me carry her."
"No," Ron denied quickly. He seemed in shock, and Bill had the feeling that he would have received a kick or a bite if he tried to remove her from his arms.
"I'll give you a hand," offered Ollivander, who clearly couldn't wait to find refuge in the house, leading him.
"Where's Harry?" Dean asked, placing a hand on Ron's back as it could help him go straight.
"Behind. Back. I don't know. He was Disapparating with Dobby. I have to take care of Hermione now," he explained with uncertain voice, staring Hermione and resting her on the couch with infinite delicacy after they entered in the house. He touched her hair with one hand and Bill, looking away from his brother for the first time, realized the pitiful condition of the poor girl. She was looking so bad that she could well have been tortured.
"She'll be fine," Fleur said with a gentle tone, after she placed two fingers on her wrist, without being noticed by Ron, to search the pulse.
This isn't how it should be. His brother was only eighteen years old and actually had the same appearance of a patient from the St. Mungo's Ward for mental illness, while his girlfriend was lying half dead in front of him and his best friend...
Abruptly realising that Harry was not there Bill turned and ran outside again, this time followed by his wife, Luna and Dean, casting an admonishing look to Ollivander as he raced past: "Don't leave him alone".
Running up the hill they all raised their wands to light the road as possible, illuminating the scene of Harry slumped on the wet ground in front of Dobby, the little elf with a knife sticking from his chest.
It could be Ron.
The thought crossed Bill's mind again, making him freeze but he forced himself to stay calm, trying to decide what to do and also suggesting to Harry to bury the elf in front of the house. Then he saw a half-conscious goblin with them.
It could be Ron.
Shortly after he saw Harry digging the grave for Dobby with surprising energy. Fleur was helping Hermione, who still was unconscious, and had already allowed the others to move after she had given them a closer look.
"I need another shovel," said Ron hoarsely, staring the outside with a weird look. He seemed old, though he was the little one. Little Ikle Ronnie who became ridiculous angry over the slightest thing, turning red as a pepper and all sulky, now was looking on with a detached face as his friend was digging a grave, his hand resting near the window trembling like it had the first time that he come to Shell Cottage months ago.
Bill turned to fetch the shovel just so he wouldn't have to see him more.
What would he have said to his mother, when he saw her again?
It could be Ron, the one with a knife in his chest, you know. Gryffindor until the end, defending his best friend. Not because of all that nonsense about the Greater Good, Ron was too simple, too honest to worry about that, just because he cared for Harry.
Bill wondered if it was strange as he feared, the fact that he was feeling jealousy toward the affection between his brother and Harry; he had never cared about his others brothers considering him a model to follow or not, so why now? And yet he wanted to be the one who Ron admired more, if only that could have convinced him to stay safe.
Ron, Dean and Harry dug the grave together, Bill, Fleur, Hermione and Luna joining them by the graveside, huddled in their coats, Ron immediately moving to help Hermione who was unsteady on her feet.
Luna was the one who closed Dobby's eyes.
"There," she said, "Now he could be sleeping."
It could be Ron.
Or Ginny.
Or Charlie.
Or Fred.
Or George.
Or Percy.
Stupid Percy.

"We'll leave tomorrow," Ron announced some weeks later. He seemed to be back to the usual Ron, only with a different light in his eyes, the light that came from adventures that Bill could not even imagine, and perhaps would never know. Dean Thomas had confirmed that Hermione had been tortured and that Lestrange spoke of a sword or something, but he did not know much, and the goblin Griphook seemed to have something in mind, something that worried Bill.
Bill just nodded in response, because he had already given Harry and the others all the warnings he could, even though he knew that they would follow their path without caring about the danger.
"Be careful," was all he said, and Ron smiled as if he was hearing something really silly.
It could be Ron.
Bill began to understand how Sirius must have felt: powerless and nervous. He hid the annoyance better than Fleur at least.
"Hey, Bill," Ron called again, "don't get up in the morning to see us. Just don't. For no reason."
"Excuse me?" he frowned.
"Do you want to keep Fleur safe? Don't get up," snapped Ron, and Harry appeared behind him and paused a moment, waiting for an eventual reply, then nodded as well.
"Bill, I don't know how thank-'
"Don't mention it," Bill interrupted him, trying to smile.
Fleur was happy as he was about not being given the possibility of getting up to say goodbye one last time, and stayed in silence until she was asleep.
The next morning, however, they were both wide awake staring at the dark ceiling, listening the muffled sounds from downstairs; when the door was finally closed, Bill felt his last glimmer of hope vanish, the hope left that came from knowing Ron was safe in his house.
It could be Ron.
For a fleeting moment, which made him feel guilty for a long time afterwards, he hated Harry Potter and the day he came into their lives.
He turned to his wife and embraced her waist with one arm, sure that she was awake. She turned on her side and hugged him back immediately.
It could be Ron.
And perhaps it would be next time.