In Fullmetal Alchemist: Brotherhood, the archvillain is none other than the scheming Father, once a tiny homunculus in a flask who dreamed big. Father tricked Van Hohenheim into granting him new powers and a bigger body, but Father's grand plan was just beginning. He also purged himself of the seven deadly sins, which included the sluggishness of Sloth.
Now in a separate body, Father's slothfulness manifested as a giant, burly man with dead eyes and a lumbering physique, and his role in Father's plans reflected that. Sloth was not a cunning spy assassin like his "sister" Lust; he was a blunt instrument, not unlike Gluttony.
Ordinarily, Sloth moved slowly and talked slowly, and he resented any work orders that he was given. He would always take the quickest route to complete a job and, unlike the sharp-tongued Envy, Sloth didn't taunt his enemies. Instead, he would lumber past anyone in his way or simply crush them, bluntly getting the job done ASAP to avoid the agony of effort. To that end, Sloth had a powerful physique that, while clumsy, was perfectly suited to heavy manual labor like digging, lifting and crushing things.
Like most homunculi, he could regenerate his body with the power of the Philosopher's Stone, though his was on the slow side. Sloth was also equipped with heavy cuffs and chains on either wrist, to use as whips or for tying things together if need be. Sloth could also charge at high speed, so fast that he can't see where he's going or control his direction. He can use this power to avoid enemy attacks or take his foes by surprise, as he did during his final battle. Sloth tries not to use this power, however, representing the sin of wasted potential.
In the story, Sloth never came face-to-face with the other homunculi aside from Father himself. Like his "brother" Gluttony, Sloth was a simple tool who was given a simple job to do, protected by Father and Pride's scheming and control of information in the world at large. Instead, Sloth worked alone and was ordered to dig a huge, round tunnel around beneath all of Amestris in secret, in order to create the world's largest transmutation circle in preparation for the final phase of Father's plan. At some point, Sloth's digging brought him under the famed Fort Briggs to the north, where he emerged to face Edward and Alphonse Elric. Sloth briefly clashed with the Briggs defenders until he was frozen, then thawed back out by General Raven so he could resume his work. Olivier Armstrong didn't like it, but there was nothing she could do when Raven freed Sloth and put him back in the tunnel.
Many of the homunculi in Fullmetal Alchemist died ironic deaths, including Sloth. His biggest battle took place inside the central government building on the Promised Day, to enforce Father's will and crush his enemies. Sloth was immune to bullets, so Olivier and Alex Louis Armstrong faced Sloth as a team. Sloth relied on his endurance and regeneration to wear the Armstrongs down, but he was pressed hard enough to resort to his super-speed. Sloth took both Armstrongs by surprise with his speed, breaking bones and smashing walls as he went. Sloth complained the entire time, resenting the need for such exertion.
Eventually, Sloth had the Armstrongs against the wall, but then Izumi and Sig Curtis arrived just in time. Izumi tossed the exhausted Sloth into the air, and Sig and Alex pummeled and impaled him on an alchemy spike, a mortal wound. To Alex's shock, Sloth was still moving, stubbornly persisting in the face of inevitable death. But Sloth's time had come, and he crumbled to dust as his Philosopher's Stone wore out. He died complaining about the effort of living and the effort of dying, and on that note, was gone. Sloth, for all his grumbling about hard work, died as a relentless champion of Father's will, and he couldn't be bothered to see the irony himself. He would probably say that such fanciful sentiments are too much work to think about.
