(plural form Genii) In Roman mythology the spirit that presides over the birth of every man and attends him throughout his life. (Women had no individual Genii, but were under the general protection of Juno. ) The Genius, as the individual life - principle, was especially honored on a man's birthday; considered as one 's higher self, it was also called to witness a man's oath. There were also corporate Genii: Genii of the family, the tribe, and the nation. Under the empire, the Genius of the emperor was worshiped throughout the Roman world. Moral philosophers, seeking to explain moral conflict within human nature, posited an opposing, evil Genius. In Christian times these two Genii became the guardian angel and his diabolical counterpart.