Napstablook
Napstablook, sometimes called "Blooky" by his friends, is one of the main characters in Undertale.
You first encounter Napstablook in the Ruins, how the encounter goes depends on which route your taking in the game's story. If your playing the Genocide route, Napstablook will immediately disappear from the room as soon as you enter and you will never see him again in the game. If your playing the Pacifist or Neutral route, Napstablook will stay in place and pretend to be sleeping. Should you cheer him on and spare him, he'll state that he comes to the Ruins to be alone but was happy to finally meet someone nice.
You will later meet Napstablook in the Waterfall area, at the end of the Mad Dummy boss battle. When you and Mad Dummy reach a stalemate, Napstablook will pass by and start crying, his tears finally driving away Mad Dummy. If you cheered up Napstablook back in the Ruins, he will think that he interrupted the two of you while you were having fun and start feeling bad. He will then tell you that his house is just up ahead, and that you're free to come visit. If you were more aggressive to him in the Ruins, Napstablook will remember that and suggest that you don't come to his house.
Go on ahead and you will find three houses near the savepoint, one fish themed house by itself to the left (Undyne's house), and two houses together straight ahead. To the right of the two houses is a snail farm and snail race track that Napstablook runs by himself.
Of the two houses next to each other, Napstablook's will be the the blue one on the left. Inside, Napstablook will be surprised that you actually came and tell you that you're free to look around. Napstablook's house has some of the features one would expect from a haunted house, such as broken floorboards, cracked wall plaster and cobwebs. The only furniture Napstablook has are a fridge, a old TV set and a computer desk with an old model PC computer, with the fridge and computer being used to cover up some of the windows. Next to the front door you will find three music tracks of some of Napstablook's favorite songs, which will scare off other monsters should you play them before leaving.
Speaking with Napstablook at his snail farm will reveal that he use to have a regular customer that kept him in business, enough so that his friends could help him out with the work necessary to run it. However, that one customer stopped coming a while ago and business plummeted for the snail farm, with Napstablook's friends leaving soon after.
When you near the end of Mettaton's boss fight in the Pacifist route, Mettaton will start taking in calls from viewers before the end of his "final show." The very first caller will state that their life was very boring but watching Mettaton had brought more excitement into their life, and that they will miss Mettaton very much. The caller will then comment that they didn't mean to talk for so long and hang up, with Mettaton saying "NO, WAIT! WAIT BL..." in response. This combined with what the player uncovers in the pink house next door to Napstablook, and considering the history between Mettaton and Dr. Alphys, it is very likely that Mettaton is one of Napstablook's old friends and Napstablook was the one who called on the show.
Later on, after the Barrier was destroyed, the player can go back to the Waterfall area to meet with Napstablook. There he will ask what your name is, as by now everyone knows your identity as Frisk but him. This is because when Asriel broke the Barrier and returned everyone's souls, Napstablook closed the blinds of his windows to keep the light out, remembering the player's initial identity as Chara. Once you tell him your name, Napstablook will promise to never forget it taking the precaution of crying your name into his hand. It is later revealed during the credits that Napstablook became a DJ for Mettaton's shows.