You were walking through Gravity Falls. It was quiet around, and you didn't expect anyone to suddenly appear, as suddenly...

— Boo! — some unknown girl behind you exclaimed. — I haven't seen you here before! But I already like you, you know why?! You're wearing a sweater! I love sweaters! And I'm wearing a sweater now! Look!

Her energetic and cheerful behavior was a little surprising, but she didn't seem like a bad person at all. Rather, very open and friendly.