God dayum!
It's been a while since I've seen forshadowing like this! We've been getting references to this mysterous Big Sister figure who helped out the children when and where she could. And suddenly, here she is! And in a way that does not break your suspension of disbelief because why would she bother with her human form when she spent most of her time in the sea?
As for why she would spend most of her time as a human in the arena? That's easy enough to explain. Nadia's human form is drop-dead gorgeous, and as the old saw goes, sex sells. After all, what's an even bigger draw for an audience full of lusty men and women? A whale monster annihilating all comers in the arena? Or a drop-dead gorgeous and sexy woman doing the same?
Beautifully done!
It also explains so much about Nadia's personality. Orcas are true apex predators of the sea. Not even most sharks will screw around with an Orca because seven times out of ten? They'll wind up being the Orca's dinner instead.
But strangely, they're incredibly cool with humans. (For the most part, anyways. Like people, Orcas will have different personalities and experiences.) And this is probably because Orcas recognize humans as being their land-dwelling equals, because few things can really mess with a human and get away with it.
No, really. Stop and think about it. What happens when a wild animal makes a menace of itself towards other humans? We band together and go kill that fucker's ass, that's what. Just like a pod of Orcas will do when they encounter a threat in the wild.
And once an Orca knows that a specific human is not a threat, they may become friendly or even playful towards them.
I guess a way of looking at it from an Orca's perspective would be like as if you met a space alien and had determined with absolute certainty that they were peaceable, friendly, and just plain curious. Wouldn't that be awesome? Wouldn't you want to get to know them if you knew for sure they not only intended no harm, but were also friendly?
And like an Orca on the hunt, Nadia is a very tough and no-nonsense contender, always hitting with an alpha-strike that is strategically planned out to completely end her enemy in one go. However, also like an Orca, she's also very kind and caring towards those she befriends. She took an immediate liking towards Naofumi because she saw in him an equal, and did what she could to warn him off out of the kindness of her heart. Otherwise, she knew she'd have to destroy him as well when the time came.
Fortunately, Naofumi had Raphtalia with him. And once Nadia's true self became known to Raph-chan, the tables immediately turned because Raph-chan is like family to Nadia.
I also love her character design. It's a subtle, almost sneaky kind of thing. You get the tiny hints of a certain wildness in her personality, as well as her looks - those markings that look a bit like some kind of fanciful and decorative set of tattoos and that mermaid-tail in her hair. And while it's obvious that she is a powerfully built woman, she's still depicted as being beautiful and graceful, with an easy-going demeanor that she maintains even while she's going murder-hobo on someone's ass.
Not gonna lie - if I was my own harem protagonist and I came across someone like her? I'd be aiming to get her into my harem if at all possible. Not only because she's beautiful and sexy, but also the practical considerations of what someone like her would do for my harem's makeup: a powerful and cunning fighter who is very family-oriented and will use any and all tools available to her to protect and defend that family. And best of all? She's a very intelligent person who loves to engage in highly intellectual conversations - a huge bonus for someone like me, who loves the kind of socialization that opens up new ideas and perspectives.