The founder and ruler of the Imperium is the god-like Emperor of Mankind, the most powerful Human psyker ever born.
However, there are other humanoid species classified as Imperial citizens, mainly mutant offshoots of genetic base-line Humans who are known as Abhumans and include such Human sub-races as the Ogryns, Ratlings and Squats. It is ruled by the living god who is known as the Emperor of Mankind. The Imperium of Man is a galaxy-spanning interstellar Human empire, the ultimate authority for the majority of the Human species in the Milky Way Galaxy in the 41st Millennium A.D. Astra Militarum Navis Imperialis Adeptus Astartes Adeptus Mechanicus Collegia Titanica Militarum Tempestus Questor Imperialis Legio Cybernetica Adepta Sororitas Adeptus Custodes Adeptus Arbites Sisters of Silence