Diamond Receptionist (1)
This monster, which has a very origami-like structure, greets the protagonist if spoken to. They inform the protagonist of how MTT Resort is great for staying at or just passing through. If the protagonist kills Mettaton, the receptionist asks about him. They say that Mettaton understood them and hired them regardless of how angular they are. At the end of the True Pacifist Route, they tell the protagonist that the MTT resort is shutting down, but they think the protagonist will still find it great to pass through.
Hand Receptionist (2)
This monster explains how the elevator to the capital is not working and claims that rooms are running at a special rate. They let the protagonist stay at the hotel for 200G. If the protagonist kills Mettaton, the receptionist wonders where he is and then stops as they feel it is unprofessional. Much like the Diamond Receptionist, at the end of the True Pacifist Route, they say that the MTT Resort is not allowing any more guests ever again.
Sad Dragon (3)
This monster appears to be unable to go home, and as such has to stay at MTT Resort. They call home to tell their family that there is some cold pizza in their hoard to eat while they are away.
Shambling Mass (4)
This monster expresses their feelings based on recent events in the game. Their emotions vary from being outraged at the broken elevator to being overjoyed about the Barrier's fall to being saddened if the protagonist kills Mettaton.
Their appearance may be a reference to The Observer from the YouTube ARG TribeTwelve, as both appear entirely black with a single, giant eye, and their head shape is similar to The Observer's hair style.
Business Manticore (5)
This monster comments on the state of the elevator in MTT Resort. They can be seen with their luggage next to them, suggesting that they were prepared to leave before it stopped working. They seem to share multiple similarities with Jade Harley from Homestuck, a webcomic for which Toby Fox composed multiple pieces of music. These similarities include the red shoes, round glasses, long black hair, and white fur covering their canine features. If Mettaton is killed, they notice Mettaton not performing for more than 5 minutes and ask the protagonist if they know what is happening.
MTT Resort Janitor
This hardworking monster spends much of their time cleaning the same spot. They do not seem to know that they create more of a mess to clean up, due to the goo that falls off of them. If Mettaton is killed, they stop attempting to clean the floor as they realize the futility of their situation. At the end of the True Pacifist Route, they are working faster as they are trying to finish up work.
Fish Receptionist (1)
This monster informs the protagonist of the actions they must go through to eat at MTT Resort. They say that even the silverware has to be reserved to eat there. When Mettaton is killed, they mention that Mettaton isn't there to press his face into the steaks and that Mettaton told them that if he ever went missing, they can use their face instead.
Snowdrake's Father (2)
This monster is the father of Snowdrake, whom he says ran away after his mother passed away. He claims to be a terrible father and is shown to want to make amends. If the protagonist kills Snowdrake, he asks if the protagonist has seen his son. At the end of a True Pacifist Route, he can be found reunited with Snowdrake and Snowdrake's Mother next to Papyrus's sentry station in Snowdin Forest. If Mettaton is killed, he mentions his boss not answering his calls, and saying that he does not want to be a comedian anymore as it is tearing his family apart. Braxton Harris created him.
This monster is rather reminiscent of a mole and is one of the workers in the CORE. They talk about their job in a rather positive manner. They seem to be a bit distraught when the barrier opens up because they lost their job right after they received a raise. They appear to remain positive and optimistic, however. They mention receiving money from an unknown source after Mettaton is killed.
This oni-esque monster complains about the amount of puzzles in the Underground, claiming that "You can hardly go anywhere without being buried in puzzles." They express joy at the thought of going to the surface and hope that there are not as many puzzles there. If Mettaton is killed, they say that they respect Mettaton for not putting any puzzles in the MTT resort.
Ficus Licker (5)
This monster forgot to make a reservation to eat and does not want to embarrass themself. As such, they stand in MTT Resort licking a ficus, in an attempt to not look like they have messed up. If Mettaton is killed, they wonder what he would think if he saw them licking one of his plants. One of W. D. Gaster's followers resembles them. The protagonist can see them in the unused room 304, room_water_mushroom.