Ranger Jackson is a downbeat Ranger who likes to do things by the book. He has the best interests of the NCR at heart but doesn't believe in taking any action unless he has gotten the orders signed and sealed through the chain of command.
Cass calls and mocks Jackson as a "washed up old fuck-up" and a "BrotherhoodScribe", strongly disliking his tendency to bureaucracy.
The Courier can state her impersonation was "spot on", should Jackson be interacted previously.
Even when you kill the Van Graffs or Alice McLafferty, he will neither arrest nor attack you.
When you talk to Major Knight, he tells the Courier that the commanding officer is in the back, so Ranger Jackson is ranked a Lt. Colonel or higher.
Even after the NCR and Brotherhood of Steel join forces nearing the end of the game and other NCR troopers accept Brotherhood armor to be worn around the base, Jackson will still attack on sight of the armor.