Captain Grey is the code name that the Spectrum Organisation has assigned to Bradley Holden.
Holden is a veteran of the World Aquanaut Security Patrol, or WASP, as is Seymour Griffiths aka Lieutenant Green.
During his tenure in the WASP, Holden served an assignment during which he held command of the Stingray, the WASP's top submarine. Unfortunately, he suffered a back injury that forced him to give up that command, and he was replaced by Captain Troy Tempest.
In consequence of that injury, Holden was confined to desk duty, which he hated.
When approached to join Spectrum in 2065, Holden served as a kind of field examiner during the exercise in which World Space Patrol Colonel Conrad Turner, aka Captain Black, evaluated him and five others for positions as the core officers of Cloudbase's senior staff. One of the reasons Holden joined Spectrum and accepted the codename of Captain Grey was to reenter the sphere of decisive action.