Lieutenant Commander Gail Purdy was a Senior Officer in the UNSC Navy. She was assigned to the UNSC Pillar of Autumn as Chief Engineer. LCDR Purdy was middle-aged and stout, with ginger-colored hair and stern and serious eyes.
She was aboard the Autumn when it arrived at Installation 04. Presumably, she took a Bumblebee life pod to the surface when the vessel crashed. At Alpha Base, she managed to survive until the very end of the event. She was one of the few specialists that was assigned guards during the ODST attack on the Truth and Reconciliation. She pointed out a conduit to McKay that she knew was vital to the function of the ship. McKay ended up destroying that very piece of technology to stop the Flood from getting to Earth.
The naval rank of Lieutenant Commander is at the same pay grade as the Marine rank of Major. Therefore, Major Antonio Silva, the leader of the ODST forces, is actually the same rank as Purdy. However, since she does not specialize in a combat field, he would assume tactical command. Also, it is never directly stated that she is Lead Engineer of the vessel, but at her pay grade, it is easy to assume that she is the highest ranking engineer aboard the ship, therefore making her the "Chief." It was likely Purdy was directly subordinate to Captain Keyes.