Noside

The animals had also perished in the catastrophe; the birds, as well as those representing the fauna of the island--all either crushed or drowned, and the unfortunate Jup himself had, alas! found his death in some crevice of the soil.

Every now and again, from time to time, the castaways are miraculously saved by some mysterious force which has provided medicine, eliminated threats, guided their discoveries, and so on. This happens from the very beginning of the story, when one of the survivors winds up safe in the interior of the island even though the rest of the survivors thought he was lost at sea. Verne is pretty good at slowly building up these mysterious miracles. At first the characters all have reasonable and logical explanations for why they happen, but as more and more threats appear and they need more help, it's fun to see it dawn on the survivors that someone or something is on the island helping them.

All that is to say, even before it's revealed that Nemo is the source of their miracles, there's a lot of build-up for him as their mysterious savior.

I'm not quite sure how a reader would react to his reveal if they didn't know who he was. There's a bit of summarizing of what happened in 20000 Leagues Under the Sea, but even then I don't know. I suppose there would have to be a lot more explanation if it were a new character, so using such a powerful old one probably seemed like a good idea. Regardless, I knew who he was and that he'd be here, and honestly I wasn't disappointed.

Nemo has, by far, the most interesting personality in the story. There's a fire and power to him, even though he's on the verge of death when he appears. And when they do first meet him, knowing he's the one that's been silently moving through the island and the surrounding waters killing and destroying and saving on the surivors behalf, the whole time, it's like meeting a god. Helped by how many times the survivors refer to him before meeting him in various, providential terms. The reactions of the survivors especially, all so cautious and wary but in awe as well; it really sells it.

The moment I especially want to highlight is when it's revealed that Nemo was unaware that the world knew about the events of 20000 Leagues Under the Sea since he presumed Aronnax and company had perished. Meaning when one of the survivors calls him Nemo, he "fixed his penetrating gaze upon the engineer, as though he were about to annihilate him." Love that, cause he probably could.

And this is also where we learn that Nemo is/was actually the Indian Prince Dakkar, who was educated and traveled through Europe, partook in the Indian Rebellion of 1857, afterwhich he became disillusioned with humanity and disappeared, becoming Captain Nemo of the Nautilus, fighting for and financing independence and rebellions around the world, and so on. Which is also why he decided to help the castaways, cause he learned they were members of the Union fighting for the abolition of slavery against the South before getting stranded.