I am nothing to you but Captain Nemo; and you and your companions are nothing to me but the passengers of the Nautilus.
~ Captain Nemo presenting himself.
YOU CALL THAT MURDER? Well, I see murder, too, not written on those drowned faces out there, but on the faces of dead thousands! They are the assassins, the dealers in death; I am the avenger!
~ Captain Nemo to Professor Arronex after sinking a ship in the Disney adaption.
Captain Nemo is the anti-heroic deuteragonist character in the Jules Verne novel, Twenty Thousand Leagues Under the Sea. He later reappears as the benefactor of the castaways in The Mysterious Island.
Very little is known about Captain Nemo's history. Only that he was once a "civilized" and "rich" man. Then for some unknown reason, he left society and built the Nautilus. It's assumed that he saw all the cruelty and injustice in society (slavery, corruption, imperialism and etc) as a crime against the world.
Once the Nautilus was built, Nemo cuts of any ends to the human world. He moslty uses resources from the sea (food, clothing, furnishing and most supplies comes from ocean). However, most of his research are based on Professor Aronnax writtings and his music are from some of the greatest composers in the world. He sinks ships to get his revenge on the world, under the philosophy that they are part of "the dealers in death".
When Professor Aronnax, his assistent Consiel and master harponer Ned Land were captured by Nemo and his crew, the Captain treats them as guests/prisoners, say that the are free to go wherever they want on the Nautilus, but not leave the Nautilus, ever.
The Captain shows the professor around the Nautilus and teaching him the tresures of the ocean. They travel with the Nautilus around the whole world. From under water forests to the Lost City of Atlantis.
Appearances in other media
He was played by the late James Mason (who also portrayed Phillip Vandamm in North by Northwest). Unlike the novel having Nemo survive as seen in The Mysterious Island, Nemo was shot and the Nautilus sank to the bottom of the sea.
Alan Moore's Portrayal
In their graphic novel The League of Extraordinary Gentlemen, Alan Moore and Kevin O'Neill made Nemo Indian as he was in The Mysterious Island.
Josie and the Pussycats
Although it is not the same Nemo from the original novels, rather than his great grandson following the legacy of Nemo, by calling himself Captain Nemo and sinking ships by a submarine like the Nautilus. But then Josie and the rest of the gang defeated and the coast guard arrested him and his gang. He was voiced by the late John Stephenson.
Captain Nemo played by James Mason in the Disney version
Nemo played by Michael Caine
Nemo played by Patrick Stewart
Nemo in The League of Extraordinary Gentlemen
Nemo in Josie and the Pussycats
Nemo played by Herbert Lom
Nemo in the original novel playing the organ
Nemo in italian animation series "The Extraordinary Adventures of Jules Verne"