Gluttony is a homunculus, an artificial being created by the entity known as Father from his own sinful desire of gluttony. His physical appearance is distinct and unsettling; he is short, obese, and bald, with oversized teeth and white, pupil-less eyes that give him a vacant, unfocused stare. The Ouroborus tattoo, the mark of a homunculus, is located on his tongue.

Gluttony’s defining characteristic is his childlike and simplistic personality. He possesses the intelligence of a young child and is incapable of acting independently, generally content to follow the orders of his more powerful siblings. His entire existence is dominated by a single, overwhelming drive: an unending, ravenous hunger. He is fixated on consuming anything and everything, with a particular preference for living creatures, especially humans, whom he finds especially tasty. This desire is so pervasive that his most common question to his fellow homunculi is whether he is permitted to eat a target or an enemy.

Despite his monstrous actions, Gluttony is not portrayed as malicious in a complex way. His behavior is more akin to that of an innocent but dangerous animal; he does not seem to fully grasp the moral weight of his actions or the permanence of death for humans. He can be surprisingly docile and even friendly when not given permission to eat, capable of having civil conversations with his captors or enemies if it does not conflict with his orders. This innocence, however, is undercut by his vicious enjoyment when he is allowed to hunt and his terrifying, berserker rages when he is denied a meal or provoked by someone he hates.

In terms of relationships, Gluttony is most strongly bonded with Lust. He is rarely seen apart from her, looking up to her as a younger sibling might look up to an older sister, and he depends on her for guidance and leadership. Her death at the hands of Roy Mustang is a profound trauma for him, plunging him into grief and fueling a single-minded, frenzied desire for revenge against the Flame Alchemist. He also shows loyalty to Father, his creator, and a general fondness for his other siblings, though this sentiment is not always returned. This is tragically illustrated when Pride, another homunculus, callously consumes Gluttony to absorb his abilities.

Gluttony serves a specific and crucial role as one of Father’s primary agents in the field. He is not a strategic fighter; rather, he functions as a multi-purpose tool. His primary utility comes from his enhanced sense of smell, which is so acute that he can track specific individuals over long distances or in complete darkness, making him an ideal tracker. He is also used to dispose of evidence, such as devouring the body of the corrupt priest Cornello or the ruins of the ancient kingdom of Xerxes, to conceal Father’s plans. In combat, he relies on his superhuman strength, agility despite his bulk, and incredibly powerful jaws to overwhelm opponents, though his lack of fighting skill and low intelligence make him predictable.

Gluttony’s most notable and terrifying ability is his "False Gate of Truth." He was the result of a failed experiment by Father to create a replica of the Gate of Truth, the source of all alchemical power. When enraged or sufficiently provoked, he can open a massive maw in his torso, from his abdomen to his lower jaw, which reveals a giant eye and extended, fang-like ribs. This false gate emits a powerful vortex that forcibly draws in anything within its line of sight, including matter, energy, and living beings, into a vast, otherworldly dimension inside Gluttony’s body. Inside this dimension is a blood-soaked void containing everything he has ever consumed, and from which escape is nearly impossible without the use of alchemy empowered by a Philosopher’s Stone. Like all homunculi, he also possesses a powerful regenerative ability, healing from almost any wound as long as the Philosopher's Stone that serves as his core retains its energy.

Throughout the story, Gluttony's development is subtle but tragic. After being captured by the Elric brothers, he breaks free when he learns of Mustang's role in Lust's death and goes on a rampage, demonstrating his raw power and grief. Later, his internal realm accidentally consumes Edward Elric and Ling Yao, inadvertently leading the protagonists to Father’s lair. After being temporarily killed and then recreated by Father, he is deployed again alongside Pride. In his final battle, his life energy is depleted by repeated fatal wounds. Pride, seeing Gluttony as a useless hindrance, decides to consume his Philosopher's Stone to replenish his own power, thus granting himself Gluttony’s acute sense of smell. In his final moments, Gluttony does not cry out for Father, but for the one person who showed him care: his sister, Lust. His end is a darkly poetic reflection of his nature, as the embodiment of gluttony is himself devoured by another.