As the commander of the Imperial Union of Planets' First Assault Fleet, Grenni came from one of the "privileged" Terran families in the mirror universe--those who gained high status in the Klingon-Cardassian Alliance, currying favor from the Klingons or Cardassians, before the rise of the Terran rebellion.
While an effective officer in his position, Grenni had "bought" his rank by using his knowledge of internal Alliance operations to aid the resistance. Stephen April considered Grenni a "grub-sucking profiteer" with no true loyalty to the Empire. He was also compared to the Ferengi of the primary universe. (Star Trek: New Empire)