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. These deaths took a heavy toll on him ... despite having tons of friends and getting love from thousands of fans, he often felt very lonely.
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Jason kept a very busy schedule, but through all of it tried to find peace by motivating his fans ... whether it be meeting in person or posting videos on social media.
Mike says he last spoke to Jason roughly a week before his death. He even got a voicemail from Jason ... saying he needed to talk, and he was going through a lot. Mike didn't realize Jason left that voicemail until after he died.