Satou Kazuma is the Ghost Trailer's leader. In the year 1999 which is 16 years before the current story, a war occurred between magicians and he tried to erase all humans by using a spell called Last Requiem. However, 15 great magicians from other Communities that opposed him combined their powers and split the world in two, thus the people of one of those worlds were erased in the Last Requiem, this world was known as The Ruined World, while The Living World remained unharmed. He is currently missing but there is a rumor that says he fell into an endless slumber as a result of casting the Last Requiem. Others say he is dead but none of these has been confirmed.
As of Episode 9, he had awoken at June 15th, 7:00 AM, which signaled the start of the Second Great Magic War.
At the end of the series he holds a picture of Kurumi Isoshima in his hand that was given to Takeshi Nanase by her, he also saves Youko Nanase from wounds inflicted by Gekkou Nanase. He also calls Youko mother signifying that what Youko meant by killing Gekkou before he can become Kazuma is that when the two brothers are sent back in time one year before the first great magician war, Takeshi & Gekkou, Gekkou eventually takes control of Kazuma's body starts the great war and the time Kazuma is sleeping for 16 years is because the body is recovering from Takeshi taking over and destroying his brother.
Also it explains why Violet didn't kill Takeshi while he was unconscious after Gekkou had impaled him because she knew Takeshi would become the Kazuma that now leads the Ghost Trailers.