Surprisingly, Napstablook is one of the least insane monsters in the Underground. He spends a great deal of his time alone, which he did before anyway (long before the Hunger set in, he had come to terms with his isolation); thus, his wanderings and empty, ghastly whispers to the walls of the more and more empty caves do little to his twisted mind.
In the first year following the Glitch, when Mettaton didn’t return from his resort, Napstablook abandoned his home and took up permanent residence in the Ruins, separating himself from the painful memories of his old, empty home. There, he hunts the small, easy prey that remains and lurks in dark corners, ceaselessly mourning his lost family. Under his breath, he constantly hums the song he never finished mixing with his cousin. He keeps his torn, ragged eyes and mouth closed except when hunting, phasing through walls without notice, neither his nor others'. Besides the muffled singing, no one can hear him coming.
Napstablook is impossible to kill, unless a monster consumes his soul. If engaged in a fight with Napstablook, he can be redirected in one of two ways. You can sing to him while trying to match the cadence of his own humming, which will drive him away, weeping inconsolably. You can also cheer for him, which he will recall as a positive motivator, and will regain enough of his sanity to remember a time when all he wanted was a friend. He will leave you be after that, fading into a wall with a few haunting notes of his unfinished melody.