I watched this anime about six years ago (2012).
From what I remember there were three sides of the story. The human race, a special organization that "protects", and a group of kids with abilities that were deemed dangerous to humans and hunted down. Sounds almost like a comic, but way better.
The protector of the special kids could transform into a monster with a split face (two different colors) and couldn't die. He was actually centuries old, and carried the hatred of what had happened to his village.
One of the kids was a girl with an incredible and magical voice. When she sang the organization could track her.
It was complicated and had a great storyline!