From Halopedia, the Halo wiki
Aaron Merrill was stationed on a UNSC Marine outpost when it came under assault by Covenant forces. During the 48 hour long battle, the Covenant sent waves of Jiralhanae soldiers on a suicide mission to devastate the marines at the base. During the onslaught, Merrill was at one point in command of Charlie Company but was soon killed in action. His death prompted the station's AI, Beatrix, to bestow the next highest-ranking marine, Sergeant Jessica Kress, with a Brevet of First Sergeant to command of the remaining troops of Charlie Company.
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