Master Chief is the only Spartan known UNSC, while Covenant contact was the only one described as a demon. It had become the name of respect by the time he was a strong enemy. Initially, Master Chief Spartan 117, a citizen of the colonies, is a human.

 His name is John that does not contain any last name. He was kidnapped at age six and secretly was taken to train for the military. He is almost faceless and has his own unique identity of green coloured armour and helmet as people have rarely seen him without that dress code. 

The first reference to why they called Spartans demons when Thel roared, “Demon, Heretic, Unholy Alien!”. His rank became correspondent with his identity as his dishonour started increasing. At that time, the myth about the Spartans proliferated the other service branches of the UNSC defence forces. 

After the fall of reach and subsequent events, John was assumed to be the only Spartan-II in order to survive. Navy personnel started calling the term ‘Top Chief’ for the other Master Chief ranked offers after that time. They give a small definition here; to their eyes, only one person was capable of earning the moniker, ‘Master Chief’. 

As the covenant are religious freaks, they consider the war against humanity to be a Holy war. There is no particular reason to define this, yet they think it holy for their unspecified beliefs. Master Chief is an uncompromising warrior from the sacrilegious enemy thus, they consider him to be a demon. 

Interestingly, covenant use misnomers for everything, so Demon could just be another technique of them twisting something to fit their belief system.