Doctor Zero is incredibly old, and has acquired many names and enemies as he pursued his ruthless agenda of balance throughout the ages. These have included kings, princes, empires, and modern governments. Not to mention other Shadows.
His most dedicated foes are The Order of Saint George: a sect of Shadow Dwellers operating under the guise of a medieval monastery. Every generation the Order empowers a human champion -a Saint George- to fight for justice and ultimately slay Zero, whom they designate "The Dragon". In this endeavor they have been unsuccessful. Zero has killed every prior champion of the order.
Zero is, or at least believes he is the oldest living organism on earth. His origin was revealed when he allowed himself to be captured by a US military intelligence agency. While seemingly comatose he projected a narrative vision of his birth from the primordial soup into the mind of a project scientist: the sole survivor of his catastrophic escape.
In the modern era Doctor Zero concerns himself with reducing human impact on global ecology. He views mankind's unchecked industrial activity as the greatest threat to planetary equilibrium. Over the course of history he has fought for balance of one kind or another. On a grand, inhuman scale his motives could be considered heroic, or at least just.
Doctor Zero has little regard for human life. He views himself as above the concerns of mundane individuals. While clinically impersonal when dealing with common enemies, he can be roused to anger by sufficiently challenging opponents, and sadistically vindictive towards those who stand in opposition to his values.
The Dragon is also capable of affection and tender emotions, as evidenced by his love affair with a human woman.
Although he enjoys select human relationships, he experiences no stress from living a highly compartmentalized life. Doctor Zero has been manipulating people for thousands of years, or longer, and has amassed wealth, property and political clout that places him among the ranks of the global elite.
Although his true nature is opaque to humans, amongst other shadow dwellers he is notorious for his ruthlessness and unrivaled power. Those who dare oppose him only do so through proxies.
Like all Shadows, Doctor Zero is biologically immortal. It is not known if he has ever sired offspring, or if his immortality will fail if he reproduces as is the case with other Shadows. He could be the progenitor of the shadow dweller race, or he could be a sui-generis superorganism which shares no relationship to other species.
Doctor Zero has the ability to generate an enormously powerful form of bioenergy which can be used to create a wide variety of effects. He can fly, surround his body with a protective field, project destructive energy beams, and produce immense blasts rivaling low-yield nuclear weapons. Using this same energy he can heal grievous injuries to himself and others. He can also permanently alter the minds and personalities of other people by changing the structure of their brains.
He can also transform his body to blend in with other species, but this presumably requires sustained effort over a long period of time. He has maintained his current physical appearance since at least the early middle ages, and conceals his identity with costumes and masks, rather than by shape-shifting.
While not invulnerable, he is superhumanly strong and tough. He also shares the common shadow dweller trait of immunity to ionizing radiation.
Doctor Zero is telepathic, and can project his thoughts into the minds of individuals over a large distance.