Mellium was the daughter of the Commander of the Ark in the 57th Segment of Time. She befriended the First Doctor and his companions when they arrived on the ship. She brought Steven and Dodo on a tour of the ship, showing them the statue under construction.
When humans and Monoids began dying from Dodo's cold, she helped defend the Doctor and his companions during their trial. She believed they could find a cure for the common cold virus they had accidentally brought. She took care of Steven when he caught the cold. (TV: The Ark)