Kouki Touga is Eita's older brother and a skilled ninja that disappeared mysteriously.
Kouki is well known for being a womanizer.
One year before Eita met Aero, Kouki went out on a "worldwide girl-experiencing trip" but never come back because he mysteriously arrived in the world of Live-Earth. Once on Live-Earth he met and saved Aero from a Gigas and later send her on Earth to recruit his younger brother. He also acquired a vast amount of metal for Asterio to work with during his visit to Labyrinth.
THAT'S WHAT HE WANTS!!! HE WANTS THE PLOT!!!!!
Kouki is a very skilled ninja that like Eita has been learning martial arts in the Touga Ninjutsu Dojo since he was young.