GASTER BLASTER SANS AND READER

(Y/n) was Frisk's older sister. She became friends with all of Frisk's friends and they loved bring around each other. So one day they went to the park but sans didn't come. Papy said that sans wasn't feeling well. But they soon found out that sans turned into a giant Gaster Blaster thing and he saved (Y/N) and Frisk's life