Mateus Callaway is a member of the Gillette Corps, a specialized unit of the Kliemann Agency. He operates as the primary pilot and driver of the unit's massive transport vehicle, codenamed "April." His duties focus on reconnaissance missions and executing surprise attacks for the squad.

He possesses a muscular and imposing physique, a stark contrast to his primary skill set as a mage specializing in summoning and controlling Fayla – magical combat creatures. His command over these entities is significant, demonstrated when he deployed a man-eating unicorn against Chaika Trabant and Toru Acura early in their journey. He exhibits confidence in his summoned creatures' strength and shows visible surprise upon their defeat.

Generally calm and collected, Mateus keeps a cool head during operations. He reveals little personal information, fostering an air of mystery enhanced by visible tattoos. He displays social awkwardness, particularly in interactions with women, a trait noted as similar to his superior, Alberic.