Cross my Heart, Hope to Die | Cross/Underverse Sans X Reader

(hello everyone! Ive wanted to do this for ages!)
Once there was a girl, she had a brother, and a best friend. Her and her best friend climbed a Mountain a long time ago... Mt. Ebbot. They fell, and soon were found by the Dreemurr family. They became the princesses of the Underground, along with being sisters now.
But, all good things must come to an end, and the girl's best friend had an idea! They ate a bundle of poisonous flowers, and soon, they had died, and so had their brother. And soon the girl's name was nothing but a myth.
Now the girl lives on the surface with the golden heart locket her brother gave her, still the most precious thing she owned. She goes to high school, but she has just arrived at new one. Will she make it through, even with the weight of self-blame and depression on her back?

(Im bad at this XD)
(The reader is short, depressed, and shy)
(cross and underverse, along with undertale, and all AUs used are NOT mine)