wow, so I'm half through the exchange fics and this one left the most impression for now. Wow. Just, I love the idea, how suddenly Nino gained his powers and how he tries to understand what's happening and all his inner conflicts. I love how he thinks it's funny at first (like in an RPG as you said), and then, when he feels the humans and realizes that a human is full of water as well, when he starts to panic. God, all those emotions. They gave me the chills.
And then the hero thing. This was wonderful and deep and so dramatic in a way that I just couldn't stop my heart from beating like crazy when I read this sentence:
He can be Jun’s superhero today.
One of my favourite moments in your story ♥

And then, there's Arashi. So caring, so heartwarming, especially when Nino understands, that they understand, that he's not alone. And Ohno's calmness was wonderful.
Also, Jun is next, isn't he? So it starts right before the 29th birthday? When I think about it, I'm craving for more, for a backstory about the rest of Arashi, how they got their power and learned to live with it, and about Jun, how things will go with him. Will read this again and again, only to stop this craving a bit. See what you did here to me, awesome author-san! :) ♥