"REALLY NOT FEELIN UP TO IT RIGHT NOW. SORRY."
Napstablook is a ghost but a cousin to Mettaton. They live in Waterfall and can be found in the Ruins. Napstablook talks in all lower case (ignoring the when he replaced the battle box with the quote above). They are encountered on any route, meaning they are one of 2 characters that can be encountered on a Genocide route that can't be killed (the other being Monster Kid). They are the Undertale Recluse. Despite not having visable hands, they do have them.
Hanging out with Napstablook in their home might involve listening to the remixes they have made, attempting to eat a ghost sandwich and lying on the floor feeling like garbage. They run the snail farm and the snail racing course. They have the recluse role in Undertale.
On a Pacifist route, Napstablook is the only one known to not get absorbed into Asriel, choosing to not answer the door to the bright light. Hence they are the only one to not learn Frisk's name and has to ask later. They cry the name into their hand so they don't forget it but cries so much that the letters run together. Once on the surface, Napstablook tours with Mettaton and Shyren.
Being a ghost, Napstablook cannot be hurt or killed but can reduce their own HP. They attack by crying acid tears. Cheering Napstablook will prompt him to show the player 'Dapper blook' which is when he creates a top hat out of tears. They appear later in the Dump in Waterfall to accidently save the player from the Mad Dummy by crying on them.