Later:
Angel took time to rest, and then she decided to leave the library to find the barrier to leave. However, she didn't want Dust to be aware of what she was planning to do, or else he might get in the way, or they might kiss again which she wanted to avoid wholeheartedly. She quietly left the library as she sneaked pass his house and continued to Waterfall, using the shadows as a way to conceal herself with a dark cape she found in the library.
As she ventured through and was amazed by the beauty that remained among the ashes, she final made it to the next place called Hot Lands. She removed the cape and hid it under a broken stand she found nearby, since this place was hot, and continued passing through. As she reached the front of a hotel, she could see that the lights inside were still on. She walked in and peered into the dining room that had a stage for shows and saw that no one there, while the opposite side of the dining room was a hallway with closed doors on one side.
After signing in relief that she wasn't spotted, she noticed an elevator ahead and pressed the button to call it down. She waited a little while, and the elevator beeped to alert that it was on her floor, as she stood in front of it. The doors opened, and Angel yelped when she saw Dust standing inside the elevator with a smirk on his face.
"Going somewhere, Angelface? Why didn't you tell me? I could have brought you here myself, instead of you trying to sneak around," Dust said, as he stepped outside of the elevator.
"I-I didn't want to bother you, so I went on my own. I wanted to see what else was down here," Angel said, taking a few steps back.
"Oh? Well, how thoughtful of you, Angelface, but I insist. It'll be pretty boring to wander around without at least someone to accompany you," Dust said, knowing that she was trying to avoid him, as he walked closer towards her.
"If it's only to avoid boredom, then it's not much of a consequence," Angel said.
"But I happen to live here, so I'd like to know where my guest is going, right?" Dust asked, as he began leading Angel along by wrapping his arm around her shoulders.
"I guess so... Hey! Where are you taking me?" Angel asked, readying to fight him.
"Relax, Angelfood cake. I'm not going to hurt you. Let's sit in the restaurant and talk for a bit, okay?" Dust said, and Angel decided to cooperate.
They sat on opposite ends of one of the round tables that wasn't broken, and began talking:
"Sooo, you've seen most of the underground already. What do you think?" Dust asked.
"It's intriguing how all of these different environments are connected, like a snowy place, a water place, and even fire with all of the lava around Hot Lands. But it seems like it's gearing towards technology with this hotel and the elevator. I'm surprised how a whole civilization lives in such a diverse underground, it's quite convenient," Angel replied.
"Wow. You've really analyzed the whole place huh?" Dust said.
"It's more of a general observation. I didn't necessarily check every place I saw," Angel said.
"Hmm, so there are some places you've yet to venture to, right?" Dust asked.
"Yes... I didn't feel right to enter these discreet locations without permission," Angel said.
"... Heh heh heh. I'm sure that's one reason," Dust said, and Angel narrowed her eyes at him.
"... huh," Dust said, with a more observing expression.
"Hm? What's the matter?" Angel asked, her eyes opening to normal.
"You have pretty eyes. They're unique too with those white highlights," Dust said with a small smile that widened a bit when he saw her cheeks become light red.
"Um, thank you. A friend of mine pointed out the white highlights after I defeated Corruption. I suppose this happened during our fight," Angel said.
"Oh, I see. What about your clothes? Did you make them like that, or did they change too?" Dust asked.
"... it was after my best friend died when I changed... she gave the last piece of her Soul to me right before she died, and my clothes changed this way," Angel said, feeling a little sad.
"I'm sorry about your friend. You two must have been pretty close," Dust said.
"Yeah... we were like sisters," Angel said, looking downward.
"... I lost my brother numerous times to the kid, but after all of the resets of trying to stop them had failed, just like with the other monsters, I couldn't let that kid kill him again... so I took it upon myself to..." Dust said as he trailed off, feeling the need to be honest with her.
"... That must have been hard for you... but I'm sure it was worse for him too," Angel said.
"... He still believed in me... even when I... he still believed in me," Dust said, partially covering his face.
"Well, you didn't disappoint. You stopped them... even though that means your brother will no longer be around, huh?" Angel said, and Dust didn't reply.
"If I may ask... do you think it's better that your brother comes back although he is killed numerous times, or is it better that you brother dies by your hands and doesn't come back?" Angel asked.
"... Honestly, Angel... I've seen him killed so many times, mourned so many more times then any normal person would... I had to stop it... even if I won't see him or any of my friends anymore, no matter what their fates always led to the same thing, so killing them... was my only option to set them free from this..." Dust replied.
"... I felt such grief when my friend died... I don't know if I'd be able to handle watching her die hundreds of times... that's just torture..." Angel said, slowly shaking her head.
"Yeah, that's exactly what it was..." Dust said.
"... I'm sorry, for what happened to you, and your brother, and all of the monsters. I'm sorry," Angel said, as she laid her hand on the table.
"... Thanks, Angelface," Dust said, as he laid his hand on her own.
They looked at each other for a moment, as Dust eyes widened and he gripped her hand.
"Huh? Dust, what's wrong?" Angel asked, trying to pull her hand back but his grip was firm.
"Angel Curtain... my Curtain," Dust said, not looking away from her.
"W-What? What do you mean your Curtain? I don't... belong to you," Angel said, still trying to pull her arm back.
Dust stood up as he tugged her along away from the table, and then he pulled her close to himself with his other arm around her waist. Angel instinctively blushed, as she tried to pull away but she stopped when Dust spoke,
"You don't remember. All of these freaking resets even made me forget... but I remember now. I know who you are, Curtain, and I know you are mine. No matter what happened, you were always mine... but something made me forget in this reset... It was Corruption. It has to be her because she wasn't above ground attacking the humans as I remember. That must be what happened. What made this specific time line different. We both did something different without realizing it," Dust said, as Angel was surprised.
"Don't you remember, Angel? You and me. We met here at Grillby's, but when the kid lost their mind, we met just outside Snowdin. I remember the first times we saw each other, and how the kid actually got us out of the underground in those early timelines. Remember? We were able to get married and live in our place in one of those timelines. Remember?" Dust asked, as Angel was still confused.
"Angel, you know me. You actually laughed at my puns and told some, you were by my side when the kid was killing and helped me during that time until... you didn't come down anymore... I thought you didn't want to come back, but now I know Corruption must have done something to stop you... Come on, Curtain! Remember me!" Dust said, with more vigor in his voice.
"I don't remember, Dust. I can't remember," Angel said, pushing away from him.
"Yes you can! You have to remember! I'm not letting you go until you remember! I don't want to be alone again!" Dust said.
'But you have me, brother. I'm still here,' His brother said to his mind.
'I know, I know! I'm sorry! I just...' Dust said to him.
'I know, brother. I understand. You want her too. You want her back... then take her,' His brother said, as he disappeared again.
"I'm sorry, Dust! I can't! I don't know why, but I can't. Unless I do, I don't know you in the way you know me! Let go!" Angel said, as she struggled more.
Dust let her go, as Angel stumbled for a moment but caught her balance.
"Why... why can't you remember?" Dust said, trying to think of any possible reason.
"I-I don't know, but keep your hands to yourself already!" Angel said, and she started heading out of the restaurant.
"Or maybe it's not that you can't remember. Maybe you don't want to! Maybe you don't want to be with me! Maybe you don't want to love me because I'm just a murderous punk to you! Even if you know why I did it!" Dust said, as he followed after her.
"Stop it, Dust. Stop it! I don't want to hear anymore!" Angel said, with her hands on her head.
"You have to hear it! You can't run away from this! You're mine, I'm yours! Come back to me, woman!" Dust said, still following after her.
"Stop talking! It hurts every time you speak!" Angel said, gripping her head tighter.
"Stop trying to fight it! Remember me, baby! Please remember me! Remember I love you! Remember you love me!" Dust said, almost pleading.
"STOOOPPP!" Angel yelled, and she started running as she jumped up and flew off with energy wings that appeared on her back.
"ANGEL! COME BACK!" Dust called out to her, but she didn't.
Dust grit his teeth in agitation as he turned away and went back to the restaurant.
"DARN IIIITTT!" Dust yelled, as he summoned a Gaster blaster to destroy a part of the restaurant to vent from his frustration.
Angel was in turmoil, as she flew all the way to Waterfall and landed where the water divided a road. She stood on her knees, as she placed one hand on her face and the other on the ground and wept. Her frustration and sadness poured from her eyes, as she started feeling the burden on her shoulders lifting. Then, as she was calming down, she remembered when she fought Corruption, and what the beast was telling her:
"Little foolish girl. You're all alone this time...
How unfortunate for you. If only he was here to help you...
The child has given me power! Enough to destroy you all without having to risk going underground...
This isn't the first time we've met, but I hope it's the last."
'... She remembered... then that means...' Angel thought, as she gasped with widened eyes when her memories unlocked and began pouring into her mind.
She remembered the underground. She remembered meeting the kid before they turned dark. She briefly remembered each timeline. But most importantly, she remembered Sans. Her Sans. How happy they were before the resets set them apart, even clouding their memories of each other after so many times.
Angel slowly stood up, as she remembered Sans, but it hurt her heart to see what he had to become to save them. She turned and ran back to the hotel to find him.
So about the Anon who asked about Story shift and Inverted fate... I know of StoryShift, so I will see what I can so with it, but I know very little of Inverted fate, so I might not do that one. If there is another AU you'd like to see, let me know and I'll see what I can do!