Nemo

 was the Captain of the Nautilus. He was sent to search for the Sixth Doctor in the furthest regions of the realm (AUDIO: The Wreck of the Titan)

He rescued the Doctor after torpedoes hit Long John Silver's boat. He sent the fairies, to help Jamie McCrimmon, Rob Roy MacGregor and Alice Lydell fight cyber-converted magical creatures. After bringing up the TARDIS from the sea floor, he was attacked by cyber-mermaids. He was almost converted but he along with Dracula and the Karkus attacked the Cyber-Planner. He disappeared when the Cybership blipped out of existence. (AUDIO: Legend of the Cybermen)

The Captain was one of the nine members of the Sisyphean Society's senior circle. He had a distaste for Hyde and dismissed Robur's Terror as vastly inferior to the Nautilus. The Master took control of the Land of Fiction and formed Martian tripods to kill all the Society members. (COMIC: Character Assassin)

Behind the scenes
Captain Nemo is a character in Jules Verne's 20,000 Leagues Under the Sea.