An assistant at the Curtises' butcher shop who is more outspoken than his employer. He has worked for Sig and Izumi at least since the Elrics initially became Izumi's apprentices. During Izumi's challenge to survive on Yock Island for one month without using alchemy and to discover the meaning behind "All is one, one is all", Mason was the monster who kept attacking the brothers. Izumi had ordered him, in disguise, to make sure nothing happened to the boys during their stay on the island.