He was 198 centimeters tall.
Holding the rank of Sergeant Major, he was the senior non-commissioned marine aboard the Q-ship HMAMC Wayfarer. He held an eight-degree black belt in coup de vitesse, and helped teach Aubrey Wanderman how to defend himself. (HH6). It is unknown whether or not Gunny Hallowell survived the battle of Selker's rift, but it seems unlikely given that the final mention of him says that Dr. Ryder & her sole remaining assistant "...had no time to spare from their desperate, losing fight to save Sergeant Major Hallowell's life..."