Clint Hill

A Secret Service agent (PROSE: Who Killed Kennedy) rode on the vehicle in the motorcade immediately behind John F. Kennedy's presidential limousine on 22 November 1963, the date of the President's assassination. Shortly before the assassination, the man was photographed with the occupants of the limousine in Dallas, Texas. He was stationed at the front of the car, alongside the driver. This photograph was retrieved from the Washington public archive in 2004 by Clive Finch, of interest to him because it featured the Ninth Doctor observing events. (TV: Rose)

Immediately after Kennedy was shot on Elm Street, the agent threw himself onto the back of the vehicle and pushed First Lady Jackie Kennedy into her seat once again (PROSE: Who Killed Kennedy) after she had "scrambl[ed] to the back of the car to recover a piece of blood and brain-stained skull". (PROSE: Wonderland) He slammed his fist into the boot lid in apparent rage and fustration as the limousine began to speed away from the site of the shooting towards the underpass. They eventually arrived at Parkland Memorial Hospital, where Kennedy was pronounced deceased. (PROSE: Who Killed Kennedy)

Behind the scenes
The identity of the agent, in the real world Clinton J "Clint" Hill, is not revealed in Who Killed Kennedy or Rose.