(Warning: Some fight parts)
They all stayed quiet as the door was being banged very hard, sounding like it was going to break. "Kid, I think you need to hide right now," said Sans, "Why?" She asked in a worried tone. "We said that nobody shouldn't really see you like this right now, and things could get worse if people get really suspicious about you," he explained. She understood as Papyrus told her to go to his room, but it was kinda too late, Undyne punched the door open, Frisk quickly had no choice but to hide behind their couch, lucky she fitted, as she hid herself, she peeked through the bottom of the couch where she saw Sans, Papyrus, and Undyne's feet, she felt cold air behind her, making her shiver, Gaster was right beside her so, he would inform her if something is going wrong or coming towards her. "Where we're you two?!" Asked Undyne in a angry voice, both skeletons stayed quiet until Papyrus broke the silence, "W...we were just having lunch, then we where about to go back on p...patrol," he said. She knew he was lying, but played along, "Oh really, well then once your both done maybe you can both continue searching on your mission," she put on a fake smile at them. "Y...yeah sure, we can do that," said Sans, that's then when she realized both of then sounded worried, she thought to herself as she saw something peeking behind their couch, it looked like a a small hand, but it immediately hid itself. She then felt a weird feeling that both of the brothers were hiding something from her, "What's behind the couch?" She demanded. "O...oh I..it's nothing Unydne, it's j...just the dog we have here," said papyrus in a shaky voice. "I'm not sure about that, I saw a hand that looked a little bigger then that and it looked like a skeleton hand," she said, "oh no..." sans began to panicked as he quickly shoved her out of the house, "Why not come back later once we are done?" He said worriedly. "Why are you two hiding something from me?" She said, "N...no we're not, it's just that... uhhh," said Papyrus thinking of what he should say. That's then when she had enough of this, she threw Sans to the wall and stomped towards the couch, Frisk was panicking, not know what to do. "Stop!" Yelled Papyrus as he pulled her away from Getting close to Frisk. "Let go of me Papyrus!" She yelled as she accidentally punched him hard by the arm, and fell to the ground. "P...papyrus I'm so sorry!" She said. Papyrus stayed quiet, "How dare you hurt my brother!" Yelled Sans, he used his magic and threw her to a wall, hitting her head so hard. "I think it's time for a Bad time Undyne..." he said in a low voice as a bone was summoned pointing at her. Frisk couldn't handle this anymore, she turned to see Gaster and told her that she should stop them from fighting, before things get more worse, she jumped out behind the couch and landed on her knees, making her hiss. "Stop!" She yelled, all of them stared at her, mostly Undyne giving a worried look at her, "Don't hurt her Sans, she didn't mean to do that," Undyne looked at her more closely, she noticed that the girl who is stopping Sans from hurting her, looked like Frisk, well similar to her. She stood up and stared at her, Frisk looked at her as started she started to walk towards her. She gave her stare, and touched her right shoulder lightly, this made both skeletons mad and thought she will hurt Frisk, quickly Sans pulled her away from Frisk, Papyrus grabbed Frisk and pulled her closely to him and gave Undyne the evil eye. "W...what's going on? Who is she?," asked Undyne, they looked at her angry, but mostly Frisk gave her a shy expression. Frisk walked over to her slowly, making sure things are okay, "Kid?" "Frisk?" Asked sans and Papyrus. Undyne didn't move, she just stared at her confused. Once Frisk was in front of her, Undyne kneeled down to her to look at her more closely. All of a sudden had a small vision from looking at frisk, she saw her as a human, then when she blinked she saw her as a skeleton with some bone figures of how Frisk use to look like back then as a human. "Frisk?" She asked, without saying a word, Frisk jumped up, hugging her around her neck. "Yes, it's me Undyne, please don't hurt Sans and Papyrus like that again!" She begins to sob. That's then when Undyne hugged her back, then looking back at both brothers, "Why did you guys hide this from me?" She asked, they both didn't know what to say, that's then when Frisk said, "W...we were trying to hide this because, since I'm a monster now, maybe other monsters won't believe if I'm still alive, and maybe things could get worse if someone told Toriel and Asgore, I'm mostly not ready to show everybody who I am right now....," she said. Undyne understood as she asked to have to private talk with Sans and Papyrus. As they agreed, they asked Frisk if she can go to Papyrus room for now until there done with there conversation. As she went to the room, Gaster followed her and closed the door behind them. Gaster sighed in relief, "If that hadn't happened, things would've gone in into the next level...," he said. Frisk was glad that she stopped them from fighting. It's been 20 minutes since they told Frisk to go upstairs, "I wonder what they are talking about," said Frisk, "I guess some plans.... or something else... I'm not sure," replied Gaster. That's then when they heard the door opened, she turned to them and saw Undyne was beside the door and Sans, and Papyrus had a few things with them, mostly safety stuff, "What that for?" She asked, they looked at each other then at her, "We are going to take a little trip to Alphys," they all said.

To be continued...
(Sorry if it went a little to violent in this chapter, this is my first time writing a book so I wanted to try and put some fights/small fights to see how it will go in this story, but anyways, hope you enjoyed this chapter and I'll be seeing you in the next chapter bye~ Chibibon)