DM wasn't always a Gundalian. He was originally a human until his main Guardian at the time, S. Dharak, convinced him that he was truly a Gundalian.
Main RP: A Mission to Remember
DM met his future Guardian, PD, on his very first Gundalian mission.
Main RP: The Aranaut Project Begins!
Soon after, he was called by Barodius for assessment on his first mission. The Emperor noticed DM's strange new Bakugan, and ordered to see it. The Emperor then put DM on yet another mission, but with only one Bakugan: A Pyrus one. If DM succeeded, he could have his other Bakugan back. Should he fail, he'd be punished.