Michael "Mikey" Klebitz is the older brother of Johnny Klebitz. He is a 37-year old Captain in the U.S. Army or the U.S. Marine Corps serving in Iraq and 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 biker lifestyle and criminal activities. Johnny responds by saying that he knows what he is fighting for. Michael and Billy Grey were friends when Michael was 15 and Billy was 13, (Johnny being only 10) and it's implied that's how Johnny knows Billy. Though criticizing him, they both seem to have a good relationship as brothers.
It's unknown if he heard that Johnny was killed in the mission Mr. Philips in GTA V.