Mabel found herself back around the corner with no memory of the last few seconds. She completed the same actions she did before, up to when she thrust the knife forward. This time the mystery person expertly dodged Mabel's attack and jabbed her in the gut. She dropped her weapon and clutched her stomach.
"Owww, hey! What'd I do to you?" Mabel groaned. The person looked at her angrily,
"Ok, ok, I thought you wanted to kill me ok? I don't even really know how to attack someone!" The person thought about it for a second then outstretched their hand, Mabel took it and was pulled up.
"So who are you?" Mabel asked. They moved their hands around in a pattern.
"What? Mabel looked at them confused. They pointed at their mouth. "Oh, you can't talk?" They nodded and moved their hands around in sign language.
"I don't know sign language, let's get to my brother, then we can tell you our plan!" The two people walked to dipper's cell where he was laying on the bed talking to Morty.
"I'm just saying," dipper was speaking. "It was just one summer when I was twelve."
"A whole summer and you didn't get anywhere with this girl?" Morty chuckled at him.
"Like you're to talk! You went a whole year right?" Dipper sat up in his bed and noticed Mabel. "Hey Mabel! Hey, isn't that one of the ones that was watching us at lunch?" He narrowed his eyes.
The mystery person looked at Mabel, then to dipper and put both hands over their mouth in a sort of X way. Dipper realized what they meant and signed to them. After a few minutes of conversation between the two, dipper mostly speaking, he turned to the others.
"She says her name is frisk, and She seems to be a good person, so I let her in our plan." He looked at Mabel, then to Morty. "She'll come with me, but she wants some of her gear when you guys go to the armoury." He drew up a list and gave it to Morty. Seconds later, they were called for afternoon role call and lunch
I know I'm going to get some backlash for making Frisk a girl, but I've always seen them as one, so that's how it is.