Mary-B300 is a SPARTAN-III Beta Company special warfare operator. She was active during the Human-Covenant War, and in particular saw action with Beta Company in several major theaters before she, along with eleven other Beta Company Spartans, were made ONI operators for the Diamond Initiative—a project which paid for more Beta Company Spartans to be augmented in Project CHRYSANTHEMUM, and created a Spartan contingent for a new Xeno-Materials Exploitation Battalion in ONI.