Sloth is the fifth Homunculus created by Father, appearing as a giant, muscular man with limited mental capacity. Throughout the story, Sloth is primarily engaged in digging an extensive network of underground tunnels that form part of Father's mysterious transmutation circle. A conversation between Greed and Lust suggests that Sloth has been digging these tunnels for at least a century. Understandably, he expresses a strong desire to finish his work so he can finally rest.
In the first anime adaptation, Sloth was accidentally created during Ed and Al's failed human transmutation attempt, taking on the appearance of their mother, which contrasts sharply with his portrayal in Fullmetal Alchemist: Brotherhood.
Upon completing the tunnel network, Sloth is dispatched to Central on the Promised Day with orders to eliminate anyone who could potentially disrupt Father's plan, notably General Armstrong. With the help of her younger brother, she manages to hold him off, but the sudden arrival of Mannequin Soldiers and Central troops complicates their strategy. Sloth demonstrates his Homunculus ability to move at supersonic speeds, nearly succeeding in killing them before Izumi and Sig Curtis arrive to assist in destroying Sloth's Philosopher's Stone.
