Umm... I'm pretty sure they explained it multiple times but I'll try explaining it. The brown haired guy was human with pufferfish gene he secreted deadly poison once he's stimulated. The twins came from family that breeds and sought after people with those traits cause that deadly poison for them is basically a drug.
Ohhh I see... so basically the family of Twins are also not entirely 'human' ... another question aren't they going to inbreed because of this. I actually didn't read I was just skimming through it.
Ooh so that's why he's called a pufferfish my dumbass didn't even process that.
What's so special about him? Why do they like him so much???