Napstablook

Napstablook is a melancholic ghost monster and musician who lives in Waterfall. They are encountered in the Ruins as the game's first miniboss.

Personality
Napstablook is a very dour individual, with low self-esteem and an introverted attitude. Despite this, they're shown to be courteous to others, such as treating the protagonist as a guest in their home, and often blaming themself and apologizing for inconveniences to be polite. Even if the protagonist previously attacked them in the Ruins on the Neutral Route, they will still treat them as a friend.

Neutral Route
Napstablook is first found blocking the protagonist's path in the Ruins, pretending to be asleep. If the protagonist cheers them up, they will thank the protagonist for being so friendly and then leave. If the protagonist fights Napstablook, they will admit that they were lowering their HP to be polite, and awkwardly leave.

If the protagonist did not fight Napstablook, they'll appear in the center-bottom basement room of the Ruins' six-holed puzzle room, They say, "I fell down a hole... now I cant get up... go on without me... wait, ghosts can fly, can't they... oh well..." They fade away afterwards, presumably to return to their house in Waterfall. (It is notable that a monster says elsewhere that it is said that only spiders and ghosts can travel in and out of the Ruins.)

Napstablook does not appear again until much later when the protagonist is fighting the Mad Dummy in the Waterfall's Garbage Zone; they interrupt the fight by crying on the Dummy until they run away. Napstablook apologizes for breaking up the "fun" the protagonist was having and welcomes them into their house, which is just east of Undyne's House. From this point on, Napstablook wears headphones.

Naptablook's house is adjacent to what is implied to be Mettaton's house, which can be unlocked with the Mystery Key sold by Bratty and Catty. In their house, you can listen to their music and they will tell you they are working on a mix CD. Napstablook offers you to eat a ghost sandwich, but you are unable to eat it and phase through it. After this, Napstablook suggests that the two of you lie on the floor and feel like garbage together, as this is a family tradition. If you lie down for a while, the music and background will change into relaxing space scenery.

Napstablook has three playable CDs: Spooktune, Spookwave, and Ghouliday. If any of the CDs are playing when you leave Napstablook's house, an event can be triggered, where Aaron and Woshua are encountered but turned off from fighting due to the frightening music.

East of Napstablook's house is a snail farm owned by their family, where Napstablook can also be found. Napstablook used to work with their cousins at the farm before they left to gain a corporeal form, making Naptsablook the last remaining worker. Here, you can play Thundersnail.

Later during Mettaton's fight, Napstablook is the first viewer to call and express their gratitude for the program, saying they watch it all the time.

Genocide Route
If the protagonist has killed all of the possible monsters before their encounter, Napstablook will simply fade away when the protagonist approaches. Later on, Napstablook does not intervene in the Mad Dummy fight as they normally would; instead the scene ends on its own, and Napstablook is not seen again.

Endings
In the True Pacifist Route epilogue, Napstablook is seen outside their house with Mettaton, having been recruited as the celebrity's sound mixer. They reveal that they were not absorbed by Asriel because when the light of Asriel knocked on their door, they just closed the blinds and continued mixing music. As a result, they initially didn't learn Frisk's name. In the credits, Napstablook is seen on tour with Mettaton and Shyren.
 * White Text - "Zzzzzzzzzz"
 * Yellow Text - " Spooky DJ "
 * In order to achieve the yellow text, they have to be cheered.

In Battle

 * See Napstablook/In Battle.

Trivia

 * Napstablook's name sounds like "Napster bloke." Napster is a defunct peer-to-peer file-sharing service that focuses on sharing music files. This references Napstablook's computer being on a "music-sharing forum" and their general affinity for music. This could also explain the similarities between some music in the game.
 * Napstablook's name could also reference the protagonist's first encounter with them. In the Ruins, Napstablook is found sleeping directly in the protagonist's path, with no other way around them. That is - Napstablook naps to block you.
 * "Dapper Blook" is a play on the phrase "dapper bloke," which is a chiefly British phrase referring to a sharply-dressed person (usually male).
 * The "Dapper Blook" top hat can be seen as reference to the Ghostly Gibus from Team Fortress 2 given it's a top hat worn by a ghost.
 * Dialogue with the Mad Dummy implies Napstablook's tears feel like or are composed of acid rain.
 * Naming the Fallen Child "Napsta" or "Blooky" will prompt the message "............ (They're powerless to stop you.)"
 * Napstablook, along with the Dummy, are the only enemies in Hard Mode that don't change in difficulty, due to missing a memo regarding Hard Mode.
 * Pathetic House, the music played at Napstablook's house, is a slowed down version of Ghost Fight.
 * By simply ignoring the light to go back to mixing their music, Napstablook is the only monster whose soul wasn't taken during Asriel Dreemurr's absorption process.
 * Napstablook's theme resembles the battle music of another RPG titled OFF.
 * Since Napstablook is a ghost, also called a 'Spectre', and OFF is about defeating spectres, this is most likely intentional.
 * As Napstablook is a ghost, they may be a reference to the so-called "ghost producers", composers who sell their works to people who can then claim them as their own.