Jason David Frank was beloved for playing the hero on-screen -- but according to a friend, his true heroism happened in real life as he battled mental health issues for years.
Mike Bronzoulis, a prized MMA fighter and one of Jason's closest friends, tells TMZ ... since they became friends in 2010, Jason was always open with him about bouts of depression and mental health struggles -- though he kept it private from most people.
Mike says Jason had multiple family members die from suicide, and his mother also died a few years ago after battling cancer.