Outside of Nissan Motor Co., it is likely few have ever heard of Greg Kelly, the board member and former senior executive jailed alongside Chairman Carlos Ghosn this week in Japan.
But during Mr. Kelly’s 30 years with the Japanese auto maker, say former executives who worked with him, he became well-known inside the company—most significantly in the last decade as gatekeeper and confidante to Mr. Ghosn, who ran Nissan for more than 15 years as chief executive officer.
Japanese prosecutors suspect that Mr. Kelly helped Mr. Ghosn understate his compensation on Nissan securities reports for the five years through March 2015. The two men, who haven’t been formally charged with any crime, are now in custody at a Tokyo jail and couldn’t be reached for comment.
Mr. Kelly, a U.S. citizen, was one of the few Americans to hold senior leadership roles at a company led mostly by European and Japanese executives. He was also the first and only American to become a senior-ranking director on Nissan’s board when he joined it in 2012, putting him alongside Mr. Ghosn and Hiroto Saikawa, who took over Nissan as CEO last year.
During a press conference Monday, Mr. Saikawa described Mr. Kelly as a Ghosn adviser and said an internal investigation at Nissan showed him to be the mastermind behind the alleged financial misconduct. Mr. Saikawa didn’t elaborate on the findings of the investigation, which Nissan brought to prosecutors.
At the time of their arrest, Messrs. Ghosn and Kelly were in Japan for a board meeting, but they didn’t arrive together, according to people briefed on their plans.
Mr. Kelly was viewed within Nissan as wielding considerable influence at the auto maker, where he at one point served as Mr. Ghosn’s most senior aide and controlled his daily schedule; former executives described him as a sort of chief of staff.
The 62-year-old company veteran was known for being fiercely loyal to Mr. Ghosn, who often turned to him to troubleshoot problems and deal with personnel matters, these people said.
“He was Carlos Ghosn’s shield. His inside man,” said a former subordinate of Mr. Kelly.
But Mr. Kelly made few public appearances and worked mostly behind the scenes, even after he was put in charge of Nissan’s corporate communications during his years in Japan, the former colleagues said.
Several described him as ghostlike. “He was very discreet, a bit of a ghost,” said one executive who worked with him.
Mr. Kelly once headed global human resources, a powerful position at a company where lifelong employment is typical in Japan and managers are often moved around the world. His other roles as head of corporate communications and the legal department at Nissan gave him some control over the information flow within the company.
Nissan said it couldn’t comment on the specifics of the monthslong investigation. “Obviously, as a company we are not taking them lightly,” said a company spokesman, who agreed with the description of Mr. Kelly as gatekeeper close to Mr. Ghosn.
Jim Press, a longtime auto industry executive who has served as an adviser for Nissan’s alliance with Renault SA, said he was taken aback by the allegations involving Mr. Kelly.
“Greg is one of the most capable, ethical executives I’ve ever met,” Mr. Press said.
After getting his law degree in 1981, Mr. Kelly worked for a few years as an attorney before joining the legal department at Nissan’s U.S. subsidiary in 1988.
He quietly rose through the ranks; when he helped orchestrate the relocation of the company’s U.S. headquarters from California to Nashville in 2006, many executives left the company because they didn’t want to uproot, the former executives said. As head of human resources in North America, Mr. Kelly hired many of their replacements.
In 2008, he moved to Japan to run the legal department for Nissan and became the head of the CEO’s office later that year, a job for which he ran board meetings. In 2009, he added the roles of global human resources chief and communications director for Nissan. He joined the board in 2012.
He would occasionally join close colleagues for a drink after work during his time in Japan, former colleagues said. He limited interactions with most staff in the office to troubleshooting problems, they said.
Mr. Kelly was a very professional, “stand-up guy,” and could be extremely cordial, but could be tough when things went sideways, said one of the former executives who worked with him.
Mr. Kelly was very close to Mr. Ghosn and worked with him around the clock, even while keeping a low profile, a former executive said.
As human resources chief, he helped his boss decide on promotions—a role Mr. Kelly’s former subordinate said made employees nervous about crossing him. He was known to sideline people who he didn’t see as team players, said former executives.
He groomed many senior executives, the former executives said, before stepping away from his day-to-day operational roles in 2015.
In recent years, he spent much of his time at his house on Sanibel Island in Florida, where he was an avid fisherman and owned a twin hull boat, according to public records and an executive who worked with him.
But he remained a member of the Nissan board of directors and a frequent presence at the company’s U.S. headquarters outside Nashville, according to another former executive.
Write to Chester Dawson at chester.dawson@wsj.com and Sean McLain at sean.mclain@wsj.com
