What might happen if Mettaton died and Napstablook took action to this? That's the idea behind Deprived Napstablook. Based on Disbelief Papyrus, Deprived Napstablook has decided to take action to his cousin getting destroyed, by trying to save monster kind from a being of genocide.
Deprived Napstablook has the design of a regular ghost, with a long body and generic black headphones. He was able to somehow get a corporeal form from Alphys, and use Mettaton's legs.
At the beginning of the game (If in a neutral or pacifist run) Napstablook is a dour introvert with low self-esteem. Napstablook cries in battle, which may indicate some form of depression, anxiety or sorrow.
In the genocide route, he becomes angry from his cousin's death. He suffers from stress and anger later on.
In the genocide route, Napstablook fades away in the ruins to his house.
Later on, after the player is able to kill all the monsters in Hotland, he presumably evacuates with Alphys. After Mettaton NEO was defeated, he was devastated and decided to take action. Initially, Alphys refused, but she later then allowed him to battle, giving him a corporeal form and a pinch of Determination.