Zentos

Zentos was Deputy Commander of the Guardians aboard the Ark in the 57th Segment of Time. He was suspicious of the First Doctor and his companions when they arrived on the Ark, believing them to be Refusian spies. He believed that the Doctor and his companions had deliberately brought the common cold virus to the ship.

He had them placed on trial for this crime, and when they were found guilty, he informed them that they would be ejected from the ship. However, when the Commander ordered that the Doctor be allowed to find a cure, Zentos complied. After the Doctor had cured the humans, Zentos thanked him. (TV: The Ark)