Michael Klebitz is the older brother of Johnny Klebitz. He's currently a US Army or US Marine Corps Captain, serving in Iraq or Afghanistan.
He e-mails Johnny during the course of The Lost and Damned. Michael's e-mails suggest he is a law abiding citizen, criticizing Johnny for his criminal activities. Johnny responds, saying that at least he knows what he is fighting for. Michael and Billy Grey were friends at the age of fifteen, (Johnny being only ten), and it's implied that's how Johnny knows Billy.