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

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In all her years of being alive on this planet, the human girl had never been so gobsmacked to stare in a mirror, only to hardly recognise the person who was staring back at her in return. Growing up, she was never really one to wear make-up. Maybe once or twice if she felt the need to try and impress someone, (as everyone tended to have that silly teenage phase after all); apart from that, make-up had never really ever touched her face. Now, she was staring back at a girl who had concealer, making her skin look just a little smoother. Eyeliner, and even a touch of eye shadow (done in a tasteful way, rather than the girls who plastered way too much around their eyes), and her lips were a slightly darker shade, being a dark red. 

Something she also had to say goodbye to was her long hair, which used to reach down to her breasts, as it was now cut into a little bob, resting just above her shoulders. Even though it had only been cut maybe half an hour ago, she was already missing the bits of hair that were now all laying around on the floor, which had broken free from her scalp with the help of a pair of scissors. Surprisingly, Chara had done a decent job. She didn't look like a clown with all the make-up, she looked fairly natural, and her hairstyle, despite the fact that she was kind of missing the long hair, she was growing to love the shorter style.

"Not bad, right?" Chara beamed, noticing the smile that Frisk had on her face, which caused Chara to have the assumption that her work had been liked by her (non-biological) sister. She was sure Sans would probably like the way Frisk looked too. That or he would say the cheesy thing that she "Looked fine the way she did", which Chara agreed, Frisk looked perfectly fine without any make-up on. She was a pretty girl even without make-up. But in this case she needed the make-up to make her look much different than her normal appearance.

"Thank you, Chara. You're a big help." Frisk replied, going to get up from her seat to hug Chara, who gladly hugged her in return. Though, after a moment, Frisk's smile slowly started to fade, as her expression transferred into a frown. "I'm really going to miss you... And Asriel. And just... Everyone." Frisk murmured, waiting for a moment to get a reply from the girl who had become a family member to her. Chara was quiet for a second, until she pulled away from the hug, still grinning from ear to ear at her older sister.

"We'll all miss you too. But we'll find some way to keep in contact. You don't have to worry. Just make sure Sans keeps you safe, okay? Or I'll have to kick his butt." She joked, causing the two girls to laugh. Frisk nodded, but knew for a fact that Sans would keep her safe. She trusted the man with her life to be completely and deadly honest; nothing would ever change that. She would trust him to the end of the universe and back, and she knew trusting him so deeply would never, ever be something she would come to regret. She just knew, deep down inside, like some kind of 6th sense. She just knew. 

After saying goodbye to Chara, many more goodbyes followed. She didn't want to miss out any of her friends and family, so when she came into the room with the others waiting for her, it was difficult not to smile when she saw their surprised faces by how different she looked. Especially Sans - his face was so priceless that she couldn't help but giggle to herself. First she said her goodbyes to Asgore, who went on about how it was a shame they hadn't gotten to know each other a little better. After all the first time they met it hadn't been for the best reasons, and over the time of her being here, she hadn't really got any time to get to know him at all. But the future always held time for such opportunities, so perhaps she would get the chance.

Then she hugged her fussy mother, who was also telling her, (like Chara), to remain absolutely safe. Toriel wished with all her heart that Frisk could remain at home with her, that she could continue to be like her child, even if she wasn't actually this human's mother. Ever since Frisk had come into their lives, she had been more than happy to be there for her as some kind of motherly figure, seeing as she no longer appeared to have one, and Frisk had happily accepted her as family. It was so upsetting to see her go, but she had to let go of the reigns, knowing Frisk was a 20 year old woman who would be well protected by the skeleton. Sans sure seemed like he would do anything to protect his sweetheart. 

Managing to finally separate herself from her mother's clingy, lovable hug, Frisk continued on to Grillby, who wished her, Sans and Papyrus luck on their endeavour, telling them that he knew they would be okay. Smiling, Frisk nodded and thanked him, while also mentioning that she hoped things continued to go well for him and Gaster. It was difficult not to laugh a little at the two monster's reaction, as they glanced to each other with shy smiles and then looked back to Frisk with a thankful demeanour. During this conversation, Gaster did proceed to also say his goodbyes, mentioning how it was a pleasure to know her, and that he was glad his son had met her. Such words really managed to touch her heart in such a way that it caused her to nod with gratitude at his kind words.

There was also Asriel, Andrew... She said goodbye to those two, making sure to apologise for all the bad times her and Andrew had, while also thanking the goat monster for being an amazing brother to her. She never had siblings growing up, so it had felt nice to have a brother and sister to talk to and hang out with, like they really were true flesh and blood. When running into Asriel and Sans that day, she never expected her life would come to this. She was happy that it did. Frisk also did not forget to tell Blair in a serious tone that she wanted her to look after Andrew, and be kind to her friend. Otherwise if she tried anything, she would not be happy. 

"Yeah, yeah. Like you could do anything anyway." Blair grumbled with a roll of the eyes and a slight growl, while she crossed her arms in annoyance towards this human girl. Frisk rose an eyebrow at the fox, as if to say "do you really want to test me?" but then Andrew went to gently place a hand on Blair's shoulder, which took the fox monster by surprise when she glanced in his direction. Then she pouted with an exhale of breath and looked elsewhere, seeming to have somehow been silenced by Andrew's stare alone. Looks like she was really willing to listen to him after all. How cute.

"anyway, we better go. i think we've dragged this out long enough." Sans mentioned, considering they were on a time limit with how long he and Papyrus could remain in these human forms. Sure, 24 hours was a long time, but what if that was just an estimate and they wouldn't actually remain in those forms for that long? So, Sans wanted to make every minute count. With a nod, Frisk said the last of her goodbyes, before going to link hands with her boyfriend, smiling widely all the while. 

"Here we go, on another adventure I guess!" Frisk giggled, earning a small chuckle and a beam from Papyrus, who actually seemed more cheerful now that they were going to leave. Hopefully elsewhere they might be able to start a better life, and they would be able to continue being together without anyone trying to get in the way of that. Though there was the chance they might be on the run for the rest of their lives, knowing Undyne might not give up so easily to try and capture them. But they would have to see.

"NYEH HEH INDEED!" Papyrus agreed loudly, while Sans nodded.

"yup. guess ya are right. let's just hope we manage to get to the location we need to get to in one piece." Sans commented with a smirk, while Frisk and Papyrus laughed at his statement. Frisk had high hopes that they would get to where they needed to go without any trouble. It wasn't like anyone was going to guess that Sans and Papyrus had somehow turned themselves into humans. You would have to be ridiculous to somehow make such a random guess. Frisk was confident.

And for a good reason.