Mark Harmon plays a good guy on his hit NCIS crime drama, but in real life the TV investigator is a coldhearted jerk with a history of chasing skirts, insiders tell RadarOnline.com.
The 65-year-old former UCLA quarterback, who plays Special Agent Leroy Jethro Gibbs on the world's most-watched drama, is far from the "fair-minded hero he plays on TV," says a source.
"Mark can be a total jerk!"
Currently, his dirt-poor sister Kristin Nelson, 72, is recovering from a serious illness in Los Angeles after being released from a Santa Fe hospital. But Mark has so far refused to visit her or to even pick up the phone.
"Kristin is nearly destitute," a source spills. "She got a small inheritance from her mother who died in 2012, but that money's virtually gone now.
"Mark is worth millions. He won't kick in a penny to help her out."
The star's bitterness stems from the late 1980's when he tried to take away Kristin's kid using the power of his pocketbook, celebrity and high-powered lawyers — only to mysteriously drop his lawsuit under suspicious circumstances.
In 1987, the former St. Elsewhere star and his Mork & Mindy wife Pam Dawber hauled widow Kristin, whose husband was pop star Ricky Nelson, into court on claims she was doing drugs around her son Sam.
Kristin vehemently denied it under oath.
She still might have lost her child, but Mark shockingly dropped the suit at the 11th hour just before Pam, who was secretly four weeks pregnant with their first child, was to be grilled about her own drug use. Pam insisted she didn't do drugs.
At the time, Kristin's lawyer claimed Mark's decision to drop the case was "unconditional surrender."
Months later, Pam was crushed by the murder of Rebecca Schaeffer, her 21-year-old co-star in the hit sitcom My Sister Sam.
She was gunned down at the door of her L.A. apartment in July 1989 by a crazed fan who'd paid a private investigator to get her address.
"Pam took Rebecca's vicious murder very hard," says a source. "To her and Mark, it was truly like a death in the family."
Mark believes death stalks him because months later, on March 15, 1990, his dad Tom, a Heisman Trophy winner and World War II hero-turned-sportscaster, suffered a massive heart attack after winning a golf tournament at Bel-Air Country Club. He was rushed to UCLA Medical Center, where he died.
"Losing his father in an instant like that, when he seemed the picture of health, still rattles him to this day," said our insider.
"For a time in 2015, at seemingly the height of his fame on NCIS, he seriously mulled calling it quits to be with Pam and their two kids."
Amazingly, family man Mark is also hiding a wild womanizing past. As a young actor, he plunged into sizzling affairs with a bevy of Tinseltown beauties. His list of conquests includes Knots Landing starlet Donna Mills, singer Karen Carpenter and Flamingo Road co–star Cristina Raines!
At the time, Mills described Mark as catnip to women, gushing: "Forget Mel Gibson. Whenever I see Mark, I think to myself, 'Your place or mine?' "
Even Mark confessed while playing rakish Dr. Robert Caldwell on St. Elsewhere, "Women would come up to me and show me their breasts and ask for my opinion. And I gave it to them!"
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