Surprisingly, Napstablook is one of the least insane monsters in the Underground. They spend a great deal of their time alone, which they did before anyway (long before the Hunger set in, they had come to terms with their isolation); thus, their wanderings and empty, ghastly whispers to the walls of the more and more empty caves do little to their twisted mind.
In the first year following the Glitch, when Mettaton didn’t return from his resort, Napstablook abandoned their home and took up permanent residence in the Ruins, separating themselves from the painful memories of their old, empty home. There, they hunt the small, easy prey that remain and lurk in dark corners, ceaselessly mourning their lost family. Under their breath, they constantly hum the song they never finished mixing with their cousin. They keep their torn, ragged eyes and mouth closed except when hunting, phasing through walls without notice. Besides the muffled singing, no one can hear them coming.
Napstablook is impossible to kill, unless a monster consumes their soul. If engaged in a fight with Napstablook, they can be redirected in one of two ways. You can sing to them while trying to match the cadence of their own humming, which will drive them away, weeping inconsolably. You can also cheer for them, which they will recall as a positive motivator, and will regain enough of their sanity to remember a time when all they wanted was a friend. They will leave you be after that, fading into a wall with a few haunting notes of their unfinished melody.