People often say that you should always remember who you are and where you came from. In 20,000 Leagues Under the Sea, Nemo sure ain't forgettin'; but he's not telling us a whole lot, either. So if you're looking for closure, well, you've come to the wrong book; it's never a good sign when the second-to-last paragraph of a novel contains the sentence, "Will I finally discover his name?" (2.23.5). But let's be honest: answers are overrated. What fun would it be if we found out that Nemo was a Spanish guy named Juan whose family was killed by some English dudes? Wouldn't you rather have your own theories about Nemo's "true identity"?
Ultimately, the specifics of Nemo's identity—his nationality, the reason he abandoned land for the sea—are meaningless. His actions speak for themselves.
Without a definite identity, Nemo can become something more than a simple human character. He can become an idea, a principle. When he cries out, "I am the law! I am the justice!" he's not kidding.