Michael Klebitz is the older brother of Johnny Klebitz. He e-mails Johnny during the course of The Lost and Damned. Michael's e-mails suggest he is a law abiding citizen, criticising Johnny for his criminal activities. Johnny responds, saying that at he knows what he is fighting for.
Micheal and Billy Grey were friends at the age of 15, (Johnny being only 10), and it's implied thats how Johnny knows Billy.
Michael is currently an US Army or US Marine Corps Captain, serving in Iraq.