Nemo and Aronnax are unusual characters, both for different reasons. In the case of Nemo, the character is remarkably mysterious. We know nothing of his past, or his interests. We know he is American, but we do not know if he has a family, or exactly where he is from. He is also a particularly cold character. When his men died, he does not resurface to bury them. He has built his submarine in order to get away from society.
Aronnax, on the other hand, is an extremely arrogant scientist. He is a marine biologist, and believes himself to be the most knowledgeable person about almost everything. This leads him to think of himself as superior to Captain Nemo, even though Nemo is the one who created the submarine. He also seems extremely concerned about his reputation, and he wants to make some big discovery in order to earn the admiration of people.