From Halopedia, the Halo wiki
"Ghost" was the nickname of a sergeant in the Hades Corps of the UNSC Marine Corps of Engineers during the Human-Covenant War. He was known for being cold, showing little to no emotion toward anything.
"Ghost" was known among many UNSC Marines due to an engagement with Covenant forces that resulted in the deaths of his entire platoon. During that incident, a dying Marine asked Ghost to "be human" for once, as he had always come across as a cold-hearted, soulless individual. The Marine then died in his arms, and Ghost "died inside", crying out to the sky and vowing to avenge his fallen platoon.
Ghost was deployed with the Hades Corps to destroy navigational data and prototype weapons during the Algolis Invasion, per the Cole Protocol. During the battle, he commandeered a set of prototype powered armor, apparently disregarding his given orders to destroy it. However, he insisted that he was performing his duty, and used the time left until self-destruct initiation to provide cover for the Marines' and civilians' extraction.
Ghost held off the Covenant forces long enough for a group of evacuation shuttles to escape but was eventually overwhelmed. His armor's shielding failed and a plasma grenade severed his arm, disabling him. Seeing that the evacuation shuttles had escaped safely, Ghost commented on his newfound humanity before uttering the code to activate the armor's nuclear self-destruct feature: "Be human." The resulting blast annihilated Ghost and the surrounding Covenant infantry.
A Marine commanding officer in orbit above Algolis edited a log of the events, redacting requests for Ghost to be court-martialed, marking him as missing in action, and stating that he had demonstrated great conduct as a Marine and as a human being. News of his success and subsequent disappearance spread quickly, as the crew of the destroyer UNSC The Heart of Midlothian were aware of his actions.
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