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NCIS has seen numerous characters come and go over 18 seasons and more than 400 episodes. But the one constant through it all has been Mark Harmon in the role of Leroy Jethro Gibbs.

With so many episodes, fans have had plenty of opportunities to get to know the character and all his quirks. There’s also a good chance that fans have missed a detail or two. Here’s everything you need to know about Gibbs — including what’s in his coffee cup.

Mark Harmon as NCIS Special Agent Leroy Jethro Gibbs in 2020
Mark Harmon on the set of ‘NCIS’ | Monty Brinton/CBS via Getty Images

The ‘NCIS’ special agent in charge is a former marine

The Naval Criminal Investigative Service is a civilian organization, but Gibbs gets his experience from the Marine Corps. Over the years, the show has revealed that Gibbs started off as military police before working his way into becoming a scout sniper. He retired as a Gunnery Sergeant before becoming part of NIS, the organization that eventually became NCIS.

During his time in the corps, Gibbs became an expert marksman and a polyglot. He’s been seen speaking Mandarin, Japanese, Russian, and using American Sign Language.

Gibbs has a lot of rules

From the very beginning of the series back in 2003, Gibbs has talked about his famous rules. It’s unclear exactly how many there are. But we do know that his inspiration for this practice came from his late wife Shannon, who mentioned a set of rules she had when they met for the very first time.

Gibbs doesn’t break his rules very often. And he’s been known to drop a rule permanently if he breaks one and things work out. One example was when Ellie Bishop (Emily Wickersham) broke the rule of “never get personally involved in a case.” She ended up discovering that Ziva David (Cote de Pablo) wasn’t dead.

In the season 18 finale, fans learned that Gibbs’ rule number 91 was, “When you decide to walk away, don’t look back.” He tried to do just that, but the final seconds of the episode revealed that he wouldn’t be walking away just yet.

The ‘NCIS’ lead character has a thing for coffee

Gibbs always starts his day off with a cup of coffee, and he’s very particular about what kind he drinks. This wasn’t really a Gibbs’ trait when the show began. But as it progressed and the writers fleshed out the character, Gibbs evolved.

He’s a straight shooter, yet a man of few words. Gibbs likes to cook steaks in the fireplace and build boats in his basement. He prefers plain black coffee that’s strong and made at home. Or, if necessary, he’ll take one from a coffee vendor. One place that’s not a coffee option for Gibbs, though, is the NCIS vending machine.

Leroy Jethro Gibbs has been married four times

When it comes to his backstory, Gibbs has had a number of ladies in his life. His first wife Shannon was his true love, and they had a daughter named Kelly. However, they were killed in a car accident and it nearly broke the marine. He spent the next several years trying to get over the tragedy.

Gibbs’ second wife was Diane Sterling – who also married Tobias Fornell – and his third wife was Rebecca Chase. They divorced because she was unfaithful, and she ended up marrying the man she cheated on Gibbs with. His fourth wife was Stephanie Flynn, who lived with Gibbs during his service in Moscow.  She left him due to his obsession with catching serial killer Kyle Boone.

In season 16, it was revealed that Gibbs had been engaged a fifth time to a woman named Ellen Wallace. An investigation determined that she was killed the night before the 9/11 attacks.

The ‘NCIS’ OG was named after his father’s friend and business partner

The name “Leroy Jethro” isn’t a common one, so many fans have wondered over the years how Gibbs got his name? In season 5, it was revealed that Gibbs was named after his dad Jackson’s best friend.

Leroy Jethro “LJ” Moore was Jackson Gibbs’ business partner at the Stillwater General Store. They first met while working in the coal mines, and both served as Marines in World War II. They eventually had a falling out after Gibbs’ mom died when he was just 14, but things were later worked out before their respective deaths.

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NCIS is available for streaming on Amazon Prime Video, Netflix, and Paramount+.