Nyarlaθotep: also known as the Crawling Chaos, is Cthulhu's right hand, cohort, attendant, belly dancer, and occasional source of amusement through torment. Adorned with strange clothing and multi-colored hair atop their pitch black skin, they are a very unique sort of existence.
Due to their currently feminine form, Cthulhu refers to them with female pronouns.
Nothing is known of Nyarlaθotep's past, but when Sam brought up how they thought themselves to be a special snowflake who was possibly neglected by their mother, Nyarlaθotep ran off crying.
When they were thrown into the Pit of Despair by Cthulhu, they exclaimed "Not again!" meaning that they were thrown in there once before and survived it. Alluding to the possibility that they may yet remain alive.
Nyarlaθotep has the default minecraft body height and slender arms. Their default outfit is a maid outfit with rainbow-colored neon gloves. In the demon world, they were a white poncho that drapes over most of their body.
Nyarlaθotep enjoys tormenting and belittling those weaker than they are, but are prone to fits of extreme anger when challenged. This fact accompanied along with them crying at Sam speaking of their mother hints that they may suffer from an inferiority complex.
Cthulhu is Nyarlaθotep's master. The two are often seen together, Nyarla performing niceties like packing him his lunch or joining him in his rendezvous with Sam. They were later betrayed by the former, kicked into the Pit of Despair. They could be heard cursing their master and groaning in anguish -- so the relation between the two is very rocky.
Sam is revered as the one who will bring about the end of the world and as such, he is blessed with the favor of Nyarlaθotep and their dark master.
Nyarlaθotep sneers at Taurtis when he attempts to destroy Cthulhu with his one-punch power. Nyarla likely sees Taurtis as little more than a tool to be used for Sam's convenience and nothing more.