The Sunshine (Undertale)

When Chara disappears, her sister looks for her. However, something happens when she searches for her sister. She falls down into a deep hole at Mount Ebott. Will she die?

No, because she refuses. She is not supposed to exist and upon entering Mount Ebott, she glitches to the time before Chara falls into the underground.

That's when she meets her first monster, Gaster.