It was a series of books about an alien race. They were very adept at martial arts, training their kids at an early age, and they would hire their services out to people they deemed worthy.
The main character and his best friend, a girl, discover something about a prospective client and wind up getting into a hand to hand fight with him. They were both young kids. The race decides to not work for this guy or his people. Later, the main character is off planet and as he's returning home, his planet is ravaged by radiation. He almost dies.
He is saved by either one or two other aliens, when they replace his skeleton with organic, living metal. He's basically turned into Wolverine without the regeneration or heightened senses. Unfortunately, that's all I can remember about the books, and I was only able to read the first 2 in the series.