Inside Nissan Motor Co. he was known as the "CEO whisperer": The chief of staff who would deliver the most delicate messages to Carlos Ghosn, and the man Ghosn would count on to enforce his directives.

Today, Greg Kelly is locked up in a small Tokyo jail cell with a toilet and wash basin, cut off from Ghosn and barely able to speak with his own lawyers. Kelly, the only American to serve on Nissan's board, was arrested Nov. 19 along with Ghosn, both on suspicion of financial misconduct. Nissan has called Kelly a mastermind of a criminal plot to underreport his boss's income.

Barely known outside Nissan circles, Kelly's decadelong role as Ghosn's loyal gatekeeper and confidant now thrusts him to the heart of the investigation by Japanese authorities. That relationship may put him in the thorny position of holding Ghosn's legal fate in his hands. Kelly, 62, could spend years in prison if it's shown he was complicit in any wrongdoing.