Sans is a MONSTER and not a HUMAN, so he doesn't follow HUMAN biology like a HUMAN skeleton. Same with Gaster and Papyrus.   

DISCLAIMER: Also, not  that there isn't usually some dark shit going on in this story, but I'm just gonna give a little trigger warning in case people get upset by it. So yeah, DARK, SENSITIVE stuff is in this part, don't get upset at me if it triggers you because I'm giving a little disclaimer here. 

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"Well, this cell definitely has more cleanliness than I would have expected." Gaster commented, as he was getting shoved into the same cell as several of the others. Because they couldn't all fit in one cell, Toriel and Mettaton were already getting pushed towards a separate cell, while everyone else was stuck in the same one. "And you've even given us all bracelets. How thoughtful of you." The scientist said sarcastically, while Undyne rolled her eye and locked the door on them. The "bracelet", was obviously around their wrist to stop them from using magic to try and escape. 

"Don't get too comfortable. You're only staying here until your asses get taken down to the real prison. Ain't nobody gonna give you a nice place to stay there." She complained, before going to take her leave. A few of the people in the group watched her leave, while Andrew was one of the people who was trying to force the bracelet off of Blair's wrist, but it wouldn't come off, it was too tight.

"it's no use. they won't come off." Sans told Andrew, who was only just wasting his energy trying to help his girlfriend. But such a subject actually made him think of something, and that's when he narrowed his eyes. He had been meaning to ask this question ever since the fight in the elevator, but so much had gone on since then he hadn't really gotten the chance. "hold on a second, i've been meanin' to ask... who the fuck took off the collar that blair was wearin' to stop her from leavin' the base? did ya do it, andrew?" Sans grumbled, glaring at the human. Andrew shook his head.

"I-I wasn't the one who took it off, though I was actually the one who... Requested to get it taken off." Andrew said, his eyes slowly trailing to Gaster which caused Sans to look at his dad too, who didn't even look phased that he had been called out. Looking at his father in bewilderment, Sans wasn't quite sure what to say. He wouldn't have expected him of all people to have been helpful to Blair. Gaster might not have really known about his past with her but... She was a terrible person. 

"why? ya do realise if ya hadn't done that, it probably would have taken fuckin' longer to use the elevator because we would have to deal with her, and then we wouldn't be in this mess!" Sans snapped at his father. Ah shit. Now was really not a good time for him to feel like a mood swing was coming on, as he didn't really even want to yell at Gaster. Perhaps that's why he continued to stare at him with that calm poker face of his, despite his yelling. Frisk noticed how worked up her boyfriend was getting, and was at his side in a moment, trying to calm him down. "ya know she's dangerous!" He continued.

"Sans... Just breathe. It's okay, I'm sure Gaster has his reasons, just calm down, okay?" Frisk softly told him. It took some time, but slowly with some reassuring, calming words, he did begin to calm down, and that's when he apologised for getting so overly verbally agitated. He didn't mean to freak out at his father. Just all of this was so fucking intense at the moment, and if they didn't get out of here soon, they would probably lose everything. Well, he and Frisk would lose everything. 

"I simply took it off because I trusted Andrew to her. I know my judgement might sound foolish, but it's obvious they care about each other." The scientist explained, glancing towards the fox and the human who suddenly found all eyes on them. Andrew nervously stared elsewhere, definitely not wanting to meet with Sans' eyes, while Blair also seemed to look away too to everyone's surprise. Maybe this whole thing had shook her up a little and shut her up as well. "She hasn't had it for awhile, actually. And yet she stuck around for him." He shrugged. 

"That is true..." Chara admitted from where she was leaning against the wall. "I noticed it too." She added. Despite the fox being a complete annoyance, she hadn't really ran away when she could have had the chance to. Even though she, in Chara's opinion, had a shitty personality, she still stuck around with that human - Andrew. Maybe she did have some form of a heart beyond that seemingly cold exterior of hers. 

"Besides, if you found me that dangerous, why didn't you just drop me?" Blair questioned, feeling Sans' eyes turn back towards her as she rose a brow. To tell the truth, she was still in genuine shock that he saved her life. But saving her life probably didn't really have many good intentions behind it. Just the whole thing of Andrew questioning if Sans would allow someone to leave Frisk behind got to him, and he decided to step in. Part of him wished he hadn't and he just left her to fall. 

"i still need god damn answers outta ya. but we ain't gonna do that here." Sans grumbled, because he didn't want to spill everything about his and her past to everyone else in this cell. Not to mention that Toriel and Mettaton could likely hear them all arguing and talking in the cell next to them, so he didn't want to yell out to the whole group about his past. The only other person who knew about it was Frisk, and he wanted to keep it that way.

"Sans, if you want answers out of her, just ask her." Chara bluntly said, crossing her arms. "Or is it because you don't want the rest of us knowing something?" She questioned, raising an eyebrow at the skeleton who could only exhale, wishing right now that this girl could keep her mouth shut and not ask questions. It didn't help that he was prone to anger a lot quicker when his moments of anger could just be extremely amplified. 

Sans stared back towards the human in frustration, until he crossed his arms and fell quiet for a couple of seconds. Did they really have to have this stupid conversation in front of everyone else? He really didn't want to. His pupils trailed around the prison cell to people like his brother, or his father who were giving him somewhat curious but supportive glances. And when he looked at Frisk, it was very clear from her stare that she was without a doubt at his side, one hundred percent. She of all people would know it was hard for him to talk about his past. He sighed once more.

"...fine." He gave in. "blair and i used to date. she's the one who toriel mentioned that one time when i was takin' frisk out on a date." Sans explained, mainly towards Asriel and Chara as they had been the other two who had been there when Toriel had mentioned something about a previous relationship. He knew from the other cell, Toriel and Mettaton were probably listening to what they were saying. But at this point he just had to stop caring. There was always the chance that this was the end of the road for him anyway... "but then... one day, she betrayed me, and i wanna fuckin' know why." He growled towards Blair.

"BETRAYED YOU?" Papyrus questioned. "HOW SO?" He said, his tone sounding both confused yet sympathetic, as it was visible to him that what Sans was talking about was bothersome to him, and it must have been a big reason as to why he became who he became before ending up with Frisk. Blair just simply listened, not even arguing back as the skeleton was getting all worked up, which was a surprise when she normally couldn't keep her god damn mouth shut. 

"back then, i loved ya. i really did. but then ya... ya just decided, i'll join some other group and throw away everythin' for that? ya just suddenly hated me... treated me like we never had anythin' special." Sans muttered, as he felt Frisk softly place a hand on his back, seeming to reassuringly rub his spine a little as she definitely felt sorry for him. She could never blame him if he did still feel sad that his first relationship went to shreds. It definitely sounded like at the time it had meant a lot to him when they had been together, and it only further showed how much of a sweet person he could really be when he decided to care about someone. 

"I know." Blair simply said back, cracking a small sad smile. "To tell the truth, I never thought of really telling you the reason why. Not once. Matter of fact, until you saved my life back there, I was pretty focused on just wanting to fucking kill you." Blair went on, before her blue pupils trailed to the floor. She didn't really want to look at anyone as she talked. Especially Sans, with that angered look in his eyes. "I guess I got so caught up in the act of hating you that... I suppose I really started to believe it myself." 

"Wait, hold on, act?" Frisk asked. "You're telling me that any time you have tried to kill Sans, or kill me, you were just acting? That just sounds..." Frisk trailed off. She wanted to say it was stupid, but then what did she really know? Then again... She herself got so angry at Blair a lot of the time that she realised she wasn't as patient as she could be with other people. She definitely got more worked up and physical in the presence of that fox. But she was just so protective over Sans that she couldn't stand him receiving any more hurt. 

"I didn't join my group. I didn't choose to go with them by my own free will." Blair admitted, looking thoughtful as she was talking. Likely because she was actually taking the time to think over her past. Sans stood, watching her carefully as he seemed like he was listening. She let out a small chuckle, but it sounded more sad than happy. That was made more obvious, as a tear actually managed to escape from one of her eyes and it ended up slipping down her furry cheek. "You really thought that out of nowhere I would just suddenly betray you? What kind of fucking person did you think I was?" Blair questioned, going to stand closer to him and poke him in the chest in anger as she stared back at him, while more tears continued to stream down her face.

"well, what else was i supposed to fuckin' think?" Sans replied, staring back at her. "how the fuck would ya feel if one day i had just turned around and acted like i hated ya? if i decided to beat the shit outta ya, and attack your soul? it's still damaged because of ya!" He snarled. A broken soul would only be a permanent reminder of what she had done to him, and it would likely never, ever go away. He would always still remember what she did, how she hurt him. Was she really questioning him like it was such a shocker for him to suddenly feel betrayed?! She had been the one he loved, and then just from nowhere, she was like an evil villain from some kind of movie or something. 

"God dammit, I know! I know that... But I... They gave me no choice... Sans, they... I was..." Blair started to get quieter and quieter with the more she spoke, and it seemed like what she was trying to say was hard for her to say. Once again, the skeleton just stared back at her, but he wasn't snapping, telling her to speak the hell up. He was still staring at her in frustration, but a mix of confusion was starting to spread in with his expression too. "...They..." She sighed once more as another tear slipped down her cheek and she stared right at Sans. "They raped me." She said quietly, slowly seeing Sans' expression contort from a look of hate, to a look of pure shock and horror. 

"Wait, they what?!" Chara cried from where she was standing, and it was clear she wasn't the only one in the room who was horrified to hear such a thing. Literally everyone in the cell who was listening stared at her in newfound dread at such a thing. It was at that moment that Andrew stepped towards Blair and grasped onto one of her hands, and she tightly held onto it like it was her lifeline. Sans was speechless at this point.

"So, I didn't leave you because I enjoyed it. I didn't hurt you because I got joy out of it. I did it because they forced me. And they said they would kill me and definitely you if I didn't listen..." She whispered. "So, I did as they told me to. And it got to the point where I started to forget who I was, and what I was doing... And what I was even doing anything for. And then I saw you, with Frisk..." Blair glanced towards the other girl, who was eyeing her in return, sadly. "And... I guess I was just jealous that you were happy. Happy without me. And because I was so lost... I still wanted to try and make you hurt." She said. "But... You're happy now. And I'm happy with Andrew." She smiled towards the human who gave her a tiny grin back. She looked back at the skeleton. "So... Now that it's all out in the open, let's just stop all this stupid shit, okay? I'm done... I won't keep bothering you anymore..." 

"i..." Sans started. However, truth be told, he had no idea what to say to all of that. For the first time in a long time he was actually seeing the person who he had originally been with. He wasn't seeing the literal monster that had taken her place and had hurt him. To be honest, maybe with such an experience she had every right to be upset and angry. It was the same with him. He got hurt, and then he became someone who just wasn't a good person. That was when he decided to just pull Blair in for a hug, which she definitely was surprised by, but she let out a couple more sobs, while deciding to slowly hug him back.

Frisk watched, beaming at the two, glad that perhaps they might have finally worked things out to at least be on civil ground with each other. It would likely take awhile for everything to feel okay between them again, and for them to become friends, but it was good to see that for once they weren't at each other's throats and that they might just get along. With the looks on everyone else's faces, it was clear that everyone was glad that the two had stopped screaming at each other. 

"I don't mean to interrupt the touching moment..." Mettaton spoke up from his cell, which was right next door to their's. "But I might just have a bargaining chip to get us all out of here." He admitted. Sans and Blair broke away from the hug, and the two let out weak chuckles as even Sans had broke down into tears for a second. He exhaled, going to wipe his tears away before speaking in reply to Mettaton.

"alright, we're listenin'."