Sloth (スロウス, Surōsu) differs greatly between the anime and the manga. In the manga Sloth is not introduced until midway through the series, where he is revealed to be a large, muscular, dim-witted man who believes everything to be pointless and tiresome. He is tasked with digging a gigantic transmutation circle beneath Amestris to be used in turning the country into a Philosopher's Stone. Though he is briefly impeded in this job when he runs into the Elric brothers at Briggs' Fortress, he is allowed to continue his work, finishing the circle in chapter seventy-eight.

In the anime, Sloth was created when the Elric brothers tried to revive their mother, Trisha Elric. Sloth is able to assume a watery composition, which she can use to drown others by grabbing hold of them. Sloth constantly suffers from memories of raising the Elric brothers. Determined to prove to herself that those are Trisha's memories, not her own, she longs to be able to kill them, as a mother could never kill her own sons. She develops a mother-son relationship with Wrath, and the two stick together during the second half of the anime. During a battle with the Elric brothers Wrath, having unknowingly merged with Trisha's remains earlier, merges with Sloth's body to help her in combat. This leaves Sloth paralyzed, allowing Edward Elric an easy opportunity to convert the water in her body into ethanol, which then evaporates into the atmosphere. Before disappearing, she tells the two Elrics to take care of each other.

Source: Wikipedia