"You see:
In this world, the elementals aren't the only guardians of the four islands, there are also some other authorities:
At the top, those who rules the world are the four counselors, the chief of the four respective clans: Kurai of the kawatake clan; Steel of the hagane clan; Haven of the Sylph clan, and Kin of the Dahrma clan.
After them come the four elementals, they are the four strongest beings in our society, They are: Kurai and Raito kawatake and hove Sylph. The fourth elemental was Steel hagane, but he left us to become the third chief of a criminal organisation called: The Ghost Network. Its a very dangerous organisation, their leaders are as strong as us, if you ever see one of them run as far as you can, you can recognize them with an eye drawing in their coat.
Below the elementals are the commanders; they're soldiers chosen by the elementals to complete difficults missions, and below them are normal soldiers: they just have the ability to control the elements..."
"And what does that have with me?"
"I want you to participate in a special selection, to see if you can succeed in it, you have great genetics, i wanna see how much will that help you, if you succeed in the exam, we'll help you go back to your home."
"What if i refuse and try to go back alone?"
"You'll just die, eaten by the sea monsters around the island."
"..."
"So, what is your choice, go to that selection and have the opportunity to grow strong and then go back to your sister; or die here alone?"