Bone-ly Ness Is Overrated (Sans X Reader)
The monsters have been liberated from the Underground by Frisk. You didn't mind the monsters, they were nice. You befriended Frisk, Toriel, Nabstablook, Sans, Papyrus, Undyne, Alphys, Mettaton and Asgor once they the monsters had settled in.
The monsters attended human history classes and the humans attended monster history classes. This idea was Frisk's idea to bring the humans and mosnters closer, but they didn't know how close it would bring you and Sans.
(Cover, undertale, and you do not belong to me)