From Halopedia, the Halo wiki
Captain Maria F. Gilliam was a United Nations Space Command officer, and the JAG officer in residence at Luna OCS Academy in 2492. Her UNSC Service Number was 00622-7120-RJ. On June 7th that year she sat on a Board of Inquiry at Luna OCS Academy, investigating a series of incidents which led to the impregnation of Admiral Konrad Volkov's daughter Inna Volkov. One cadet questioned was a young future-Admiral Preston Cole.
It is not specified whether Maria was a Marine Corps Captain or a Navy Captain.