Summary
UNSC Marine and ODST. Waller was one of the soldiers stationed aboard the Pillar of Autumn as part of First Lieutenant Melissa McKay's unit. He made the jump to Installation 04 along with the rest of his unit when the ship crashed after being attacked by the Covenant.
Presumably, he was part of the attack that took Alpha Base and later part of the convoy McKay made to transport supplies from the Autumn.
He was the ODST in charge during the amphibious assault on The Silent Cartographer. He and his men came under heavy Covenant attack, but they managed to pull through, and were later picked up by Foehammer aboard Echo 419.