William Ryan, mostly called Colonel, was a Colonel in the United States Marine Corps.
William Ryan served as an instructor at SERE (Survival, Evasion, Resistance and Escape) school for three years. Lieutenant David Cameron and Gunnery Sergeant Leroy Jethro Gibbs were under his command during Operation Desert Storm.
Cameron died in Gibbs's arms after saving his life, with Ryan right there next to them but at some point, twelve years later and unable to cope with Cameron's death being a reality, he began hallucinating that Cameron was still alive which ultimately grew into full-blown paranoia schizophrenia.
Two years later, Ryan was serving in Iraq, where he found $30,000,000 (in US currency) of Saddam Hussein's cash. When he investigated the irregularities in the counting, his reports to higher HQs disappeared or were ignored and shipments were redirected at the last minute.
As such, Ryan suspected a conspiracy was behind the whole thing. He set a decoy, one vehicle and $2,000,000 in cash to draw the "conspirators" out. It did, just not the way he intended. They killed his unit and captured Ryan. But he didn't know who they were. He killed one of his captors, and escaped, along with "Cameron".
As, Ryan and the cash were officially reported MIA.
Three weeks later, he was spotted by FBI re-entering the US under another name. He made the FBI's Most Wanted List, forcing NCIS Director Thomas Morrow to inform Gibbs who had since become an NCIS Special Agent and Gibbs's colleagues, Agents Anthony DiNozzo and Caitlin Todd of the development which left Gibbs shocked.
Knowing that the FBI were hot on Ryan's tail, Gibbs took matters into his own hands and began pursing other leads that he hoped would lead him to his former C.O.
Gibbs soon discovered that a man named Mike Granger was the one responsible for the supposed conspiracy in Iraq and that Granger had also killed the FBI agent Carlson whose body Gibbs and the team had discovered at Ryan's cabin which had prompted FBI agent Tobias Fornell to get involved.
Eventually, Granger was arrested although Ryan was left shaken when it was discovered that the "Cameron" who was accompanying Ryan was actually an hallucination, the stress of which caused Ryan to suffer a psychotic break.
As a result, Ryan was taken to a mental hospital for treatment when it was later revealed that Ryan had been diagnosed with paranoid schizophrenia and as such, would be receiving treatment for his illness.
- Terry O'Quinn who plays William Ryan previously appeared in NCIS's parent series, JAG as Captain (later Rear Admiral) Thomas "CAG" Boone. In addition to that, O'Quinn currently has a recurring role in the reimagined CBS series, "Hawaii Five-0" where he plays Commander Joe White, the former Commanding Officer and mentor of Lieutenant Commander Steve McGarrett.