Of course by the time I fought Papyrus I just wanted to be his friend. I still want to cry a little at how perilously close he comes to admitting how lonely he is only to heap it all on Frisk! Then comes Waterfall, and a drastic change in mood.
Waterfall really got to me. The eerie chimes of the "Waterfall" theme haunt me to this day (curiously K. once told me it was music that shi found restful), and the blue-tinged dark, the phosphorescent flowers and crystals, and the records of the monsters' wistful desire to see the true night sky again...and memories of the terrible war with the humans that nearly wiped the monsters out.