After-Heart

Everyone knows that Heartless are the main antagonist to the Kingdom Hearts series. But how does one exactly become a heartless? Is there a process between losing your heart and becoming a Heartless? Can one lose their heart and still be good or more importantly, can they get it back?
Mitsuko is a teenage girl that lost her heart in a tragic accident and is in the process of figuring out what is on the other side for her. Will she become a Heartless or is there still a chance for her?