A/N- This chapter is part of a multi-chapter arc that discusses mental and emotional trauma and their effects.

"I... I do not know what to do." Toriel kept her voice low, to try and ensure Frisk didn't overhear. Some of her guests were less adept at being quiet, but they were trying.
"Frisk's always had nightmares."
"They have? I did not know that- they did not tell me."
"Yeah, they don't like to make a big deal of their own problems." She wished that Sans was making jokes or trying to get a rise out of someone- herself or Papyrus being his most common targets- but he was leaning against the table, his arms folded, clearly treating the situation as too serious for his usual levity. She mused that he often reacted that way to her child- all their friends clearly viewed them as important.
"Come on- the kid might be the toughest person I know, in their own nerdy way." Undyne snorted. "What've they got to hide from?"
"I, um." Alphys fidgeted; all eyes turned to her, which made it harder for her to get the words out. "I- I think that, um. M-maybe it has, something to do with, uh. Just being anxious, maybe? I mean, we don't know much about their past, do we?"
"Aha! That is something that I, Papyrus, can help with!" His pleasure at being able to help with the puzzle of Frisk's mind made him... exuberant, and Toriel begged him to lower his voice. He tried- turning it instead into an overly-dramatic loud whisper.

"I know," he said, "that Frisk used to sleep in something called a garage!" He beamed, clearly thrilled with his inside information. The rest of the group were underwhelmed.
"Wait, that's it, Papyrus?" Undyne stared at him. "Do you at least know why?!"
"Yes I do!" He stood a little straighter. "It was because they were homeless!"
"Which was a complete mystery to everyone before now," Sans remarked. Papyrus nodded emphatically, completely missing the sarcasm.
"It's a good thing that I remembered that, then! That- is useful information."
Toriel groaned and rubbed her face. "But why would they be so much worse now? They should be getting better, should they not? They have a family, friends, a home- now we have a real home. I do not understand."
"Um. Maybe it has... something to do w-with leaving the Underground?" Alphys wrung her hands. "M-maybe they... associated the Underground with, uh, with that, that stability?"
"Whaaaat?" Undyne flopped dramatically on the table. "They were so set on getting out and now they're pining to get back in?! Urrrrgh, that punk..."
"I d-don't know, just, maybe?"

Sans suddenly shifted. "Actually... I asked Frisk if leaving was really worth it, considering what they had Underground. They said they "didn't belong there"." He shrugged. "Seemed they didn't like being trapped."
Alphys frowned. "Do you think- maybe- they felt like they wanted to be there, but... but sh-shouldn't? D-despite what they wanted?"
"Hell if I know. It's one theory. I do know the kid has issues, and they'd rather put 'em all aside for someone else's." He snorted, remembering their recent borderline argument. "No matter if they deserve it."
"I believe in Frisk's judgement!" Papyrus announced. "If they believe someone deserves their compassion, then so do I! They have excellent judge of character- after all, I am one of their closest friends!"

"Yyyyeah, but that's not the point, is it, Papyrus?" Undyne sat on the table, ignoring Toriel's protests. "The point is them ignoring what they need. I mean, when was the last time they were actually selfish?"
Papyrus gasped dramatically. "You're right! Everything they do has been for us! We must give as much in return!"

Toriel sighed. Alphys and Sans were both very helpful- she had suspected they would be, but trying to invite them without Papyrus and Undyne had been impossible. There was no doubt that the other two were every bit as interested in Frisk's well-being- they were certainly more outwardly passionate about it. But they weren't as good at understanding how the child thought. Alphys and Sans, however, both had quite the golden touch with her child.
She looked helplessly around. She knew so little about Frisk, even after living with them for... Goodness, nearly a month now. They were still enigmatic, often distant or distracted, and very reluctant to speak in detail about their past- beyond the occasional hint they dropped. And none of them- not even Sans- had managed to get them to talk much about what had happened with the barrier.

Not that any of them had tried overly hard in that particular regard. Whatever happened had clearly hit them hard, and her wishes to give them space had been respected. But was it possible that had something to do with it as well? She mentioned it- but nobody had any sort of solid theories there, since nobody knew what had happened.
Alphys suggested they all take it in turns to spend time with Frisk- just focus on seeing if they were willing to open up, and what helped calm them down. There was no question as to who would go up first- all eyes turned on Sans as soon as she mentioned the idea. He shrugged and headed up to Frisk's room.

He knocked and entered. The curtains were open, and sunlight streamed through. Lacking a desk for the moment, Frisk was sat on the floor, surrounded by paper, sketchbooks, and coloured pens. Sans only caught a glimpse of what they were working on before they shuffled it out of sight, but it wasn't the sort of work they usual did. It wasn't drawings of people or places- it was abstract, heavy with black ink, and seemed full of sharp edges and eyes. Sans frowned and sat next to Frisk.
He hadn't seen them in a couple of days, not since they'd moved up to the surface. The difference was astonishing. Their skin- which had become healthy from regular visits to the surface- was pallid, their eyes bloodshot and there were shadows under them.

"Kid," Sans said, softly. Frisk didn't look up, busying themself with tidying their art supplies away. "Kid, when was the last time you slept properly?"
They paused. Then they shrugged and pushed everything to one side, finally turning to face him.
"Did Toriel send you to try and get me to go to bed?"
"No, buddo. Nobody sent me. But- come on, Frisk. It's obvious something's up." They turned away; instead of pushing, Sans just sat there, next to them. When they finally spoke, their words took him by surprise.
"Sans? What... What are you scared of?"
He stared. "Where'd that come from, kid?"
"W-well, I mean, everyone's scared of something, aren't they?" The question was almost pleading in tone. "E-everyone. Right?"
"I guess they are." The kid had backed him into a corner with their words. Dammit, but they had a way of doing that. He didn't want to talk about it, he didn't even want to think about it, but he couldn't shut Frisk down either.

They finally turned back to him, and he was struck by how vulnerable they were. This was the kid who had ventured throughout the Underground, righting wrongs and fighting battles with the strongest of monsters. He remembered when they'd first met- when they had turned to face him, fear in their face- but defiance had been in their eyes, and they hadn't flinched from him. That fierce courage in the face of their own fears seemed to have faded.
"You want to know what I'm afraid of, kid?" He spoke faster than usual, before his doubts got the better of him. "I'm scared of losing this. I'm scared of losing... hope." There was so much more to it but no more words came out, and he wasn't sorry for it.

Frisk stared at him for a long moment, before dropping their head. "I know."
"Come on, kid-"
"You... don't remember." They shrugged, trying to smile at him through their tangled hair. "Nobody does. What- happened at the barrier. You- you mentioned it before, and..."
"You want to tell me about it?" Frisk shook their head, hard, then steadied themself. "Well, you want to tell me why you look like you haven't slept since then?"
"Um. I haven't, much. I mean, I try, and then... I'm guessing Toriel told you. I... didn't want to worry anyone."
"Well, we are worried, kid."

Frisk sighed. "I guess... Part of it, is... I'm scared I'll wake up alone. If I wake up at all. Or that I'll be... worse than alone. And- and there'll be no getting out of it, that I made all this up to make it seem less hopeless. I just..." Frisk hugged themself. "I want this to be real. If I'm awake it's still real, isn't it? But-"
"When you go to sleep, anything can happen, and it feels more real than this." Sans knew. And as Frisk looked up at him, nodding, he knew that they had a good idea of how much he understood, too.
"You're not alone, bucko. You never have to be alone again, if you don't want to be." He stood up, putting his hands out to Frisk. Confused, the kid let him pull them to their feet.
"Go and change for bed, kid." They flinched. "I'm going to be right here, okay? I won't leave this room until you're up. But you need sleep."

Frisk paused, then agreed. All of the anxious energy that seemed to have been keeping them going suddenly dissipated, and they slowly grabbed their pyjamas and left to change.
While they were out, Sans shuffled the drawing supplies to one side and started to undress himself. He'd been trying some new clothes out, something new, and part of it involved trying what some monsters- and apparently, most humans- did and wear shorts under his shorts- or, in this case, shorts under his jeans, something else humans apparently often wore. They were comfortable enough, and he figured he could sleep in them and his t-shirt.

His socks joined the pile and he attempted to straighten Frisk's duvet- clear evidence of the child's restless sleep. It was far more effort than he usually put into his own bed, but this was important. He needed Frisk to get into the bed without automatically thinking of the nightmares.

When Frisk entered the room, though, he wasn't sure they were thinking of much at all. They were having trouble keeping their eyes open; their button-up pyjama top only had a couple of buttons fastened and they were done up wrong. They dropped their clothes on the floor- not a habit of theirs- and sleepily crawled into bed as Sans held the duvet up for them. He tucked them in, then shut the curtains with a whisk of magic.

"I like it when you do that," Frisk said, so quietly he could barely hear it. Then, "Sans, do you..." Frisk let out a jaw-cracking yawn, "do you 'member... when you read me that story?"
"You want a story now, buddy?"
Frisk shook their head, curling up close to him, so their head was leaning against his shoulder. It didn't leave him much room to move, but Frisk was asleep in seconds, so that was fine.

Sans was starting to doze himself when the nightmares started. Frisk started twitching, jolting him awake; then came incoherent cries and half-formed words. They were clearly on the verge of waking themself up.
Sans grabbed one hand, making the sorts of noises he used to make to get Papyrus back to sleep- when such a thing was possible. With his other hand he brushed the hair out of Frisk's face and used magic to untangle the duvet from their legs. The sounds subsided into something more like soft sobs, then faded further. Frisk opened their eyes. An inventive stream of curse words went though Sans' mind; but they didn't really seem to be fully awake.
"Sans?" they slurred, barely discernible.
"Right here, Frisk."
"Don't leave me."
"I won't. Go back to sleep. I'm right here."

To Sans' relief, Frisk did just that. Half an hour later, all was still when Toriel stuck her head around the door. She came over, far quieter than one would expect for her size, to see how Frisk was.
Sans whispered that he'd have to stay there until Frisk woke up. She nodded and left, returning with some books; placing them on the bedside table, where Sans could easily grab them with magic if he needed to. Then she left, with one last, long look at the child, finally deeply asleep.

There were two more nightmare episodes, neither as violent as the first; Sans was able to get Frisk calm without them showing signs of waking, and after the last, he found that Frisk could use him as a pillow, after all- regardless of what he had planned. He spent a little time lying on his back with their head on his shoulder, reading by the light of his own magic as he levitated a book above him; before finally, emotionally drained himself from Frisk's panics, he let himself fall asleep as well.
He was blissfully unaware of Alphys taking pictures of the pair of them, or of Toriel dropping off two plates with pie on it (a new recipe Frisk had picked out before the move, heavy with exotic fruits). He wasn't aware of much until he felt Frisk stirring. The movement catapulted him into wakefulness, and he watched them until they yawned and rubbed their eyes.

"How are you feeling, buddy."
"Mmf." Frisk buried their head under the duvet, but he still heard the reply. "Better, I guess."
"Yeah? Slept okay in the end?" He saw the duvet shift as they nodded. "I think Alphys was clamouring for your company once you were feeling better."
"Oh." Frisk frowned, lifting their head from out of the duvet, their hair a wild mess. "Um, do you think... If I slept over... Would she make me sleep in another room?"
"You'd have to ask her, kid. You don't want to sleep alone, I take it?" Frisk shook their head, ruffling their hair even more. "Well, between the lot of us, I'm sure we'll work something out."

This was a difficult chapter to get to feel "right"- trauma affects people in many ways, and I feel like Frisk would definitely show signs of that and that it would need to be addressed.
 I didn't want this to turn into a massive "deal with Frisk's mental health" story, but even if we overlook Frisk's past, what they went through in the Underground would have a massive psychological impact on ANYONE. I really felt that it was important to me to examine that. This is the first of a five-chapter arc focusing on that.

Hopefully I've managed to deal with this complex and varied issue in a realistic and reasonable way; if anyone feels otherwise, let me know.

-Ren