Catherine Elizabeth Halsey, M.D., Ph.D., (Civilian Consultant 409871) is a praised scientist and employee of ONI, best known for her work with the SPARTAN-II super soldier and MJOLNIR armor programs. Despite being a civilian, she held considerable authority within ONI and commanded considerable respect from many military figures. Publicly, she was well regarded, in particular due to the high esteem she felt for her Spartans and guilt she endured from condemning them to the life of a super soldier.

Appears in "The Package" and "Homecoming".

(Source Halopedia)