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Mission aboard Mona Lisa
Percy was part of Sergeant Lopez's squad sent to investigate the prisoner transport Mona Lisa in Alpha Halo's debris field. Unbeknownst to the Marines, they were acting under orders of the Office of Naval Intelligence, which had been using the Mona Lisa to conduct research on the Flood. Boarding the ship using a Pelican dropship, Percy and the other Marines were attacked by a group of several unarmored Covenant Sangheili in the ship's cargo bay. After the skirmish, Percy was amused by the Sangheili's lack of armor, jokingly suggesting that they were "running out of money".
Percy then accompanied Lopez's team when they discovered Rabbit's body, smashed into pulp by a Sangheili. Soon after, the Marines received new orders from the Red Horse to purge the Mona Lisa's navigational databases in accordance with the Cole Protocol. The Marines split up into two teams, and Percy was assigned to Lopez's team along with Mahmoud, MacCraw, Rakesh and Singh. Later, after ONI operative Major John Smith had betrayed the team, Percy helped apply bandages to a wound Sergeant Lopez had received when Smith escaped. Continuing toward the bridge, they made their way into a long, dark corridor, which Percy illuminated by throwing flares. Here, they were attacked by a Flood-infected Ayad, as well as a Sangheili combat form. After both Flood forms had gone down, the Marines were caught by surprise as the Ayad combat form recovered and attacked them again. Percy, frantically firing his rifle and cursing as he did so, was hit as the Flood form swung its appendage at him, breaking his neck.
Along with a number of other Marines, Percy is not present in the motion comic adaptation of The Mona Lisa. His line about the Covenant "running out of money" is spoken by Orlav instead.
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