Black Goo

Black Zetsu was physical manifestation of the Devil's will. It secretly instigated many of the events that shaped the world. To further it's plans, It partnered with the evil warrior who was the most loyal to him.

Backstory
Before the Devil was about to be sealed away by Elyus (alpha god), He created a will that facilitate his revival by secretly manipulating anyone into reviving him. As a result, He roams around the world to search for the most loyal person.

Personality
Black Goo had the skill of appearing extremely trustworthy, obedient and loyal, being entrusted with the villain's secrets. In actuality, however, its true loyalties were kept secret from nearly everyone, including the evil warrior who was the most loyal to him himself.