Gravity Falls X South Park: Calamity Story: Fox's senses as a seeker sensed no good in another dimension, so she asked Fatal Error to send her a portal to where the trouble was (not knowing it was South Park [which is why the portal and her eyes are red). She saw Bill Cipher, remembering that he always calls her by her symbol [The Amongus].
note: yes, she dyed her hair pink and she decided to wear her pink sweater. the color of her eyes is red because she's been sent by Fatal Error.
