Imaginary III Sans x Reader III

"Frisk, who are you talking to?" Sans asked. "(Y/N)." Frisk replied. "Is she some imaginary friend?" Sans asked again. "I assure you Sans. (Y/N) is very real." Frisk replied. "Believe what you want kid." Sans said as he left the room. Frisk sighed and looked to the (F/C) mist. (Y/N) nodded sadly. Only Asriel and Frisk could see her.

When would it change?