First Mate

 was a being from another universe who entered the Doctor's universe due to the Nevermore's benchmarking. His very presence drew life from its surroundings which killed part of Rostrum's vines, and weakened the First Doctor, Susan Foreman, Ian Chesterton and Barbara Wright. He was trying to sabotage the ship to protect his universe. He sacrificed himself to close the holes. (AUDIO: Here There Be Monsters)