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UNSC Marine Corps
"Blue Leader" was the designation of a UNSC Marine serving during the Battle of Earth.
Following the Battle of Cleveland, the Pelican dropship he was carried on picked up Ruwan Ackerson, whose driving of a transport hog made them pick him up. Ruwan told him of the Key of Osanalan, and what it was - an imaginary object invented by him and his brother when children. Blue Leader judged that if the Covenant were readying their full-scale invasion of Earth, then Mars - where Ruwan's brother, Colonel James Ackerson was stationed - had fallen to the enemy.