In Fullmetal Alchemist: Brotherhood, brothers Edward and Alphonse Elric begin their journey as a way to fix the damage from their attempt at human transmutation, but things end up growing more complicated from there. They get dragged into Father's insidious scheme, and he's got seven homunculi working for him.
Lust was one of the first three homunculi to appear, right alongside the shapeshifting Envy and the all-devouring Gluttony. Lust is the brightest of those three, and she has abilities no other character can match.
Lust's Powers & Methods
Each of the homunculi is based on a cardinal sin that Father purged from himself, with the shadowy Pride being the first (and borrowing Father's original image). At some point, Father rid himself of his longing for human flesh, taking the form of Lust. Lust is the lone woman among the homunculi, and she makes sure her appearance has a strong effect on the humans she wishes to manipulate. Lust only feels minimal carnal desires about other people; more often, she uses her good looks to inspire lust in others, allowing her to control just about anyone. She can't shapeshift like Envy can, but neither does she have to. Her body and her good fashion sense are enough since she is willing to wear human clothes to fool people
Lust is also a squad leader and assassin, and Father often trusts her to lead squads of three homunculi on missions. She ranks third among the eight homunculi, after Father and Pride, and Envy and Gluttony are ready to follow her orders without question or complaint. What is more, Lust always seems to know what is going on, and she is quick to follow developing leads to see if Father's plans are in danger. The dust barely settled on Edward's battle with Cornello when Lust arrived with her gang, and she finished off Cornello for good. When Ed broke into Laboratory 5 to look into the Philosopher's Stone, Lust was soon on the scene, bringing Envy with her, making to keep Father's plans safe without killing Ed in the process.
Lust has an ability called the Ultimate Spear, meaning she can rapidly elongate her fingers into flexible spikes, seemingly without limit. These "spears" are very sharp and pointed, and Lust can pierce absolutely any material with them: bodies, wood, stone, metal, whatever. Lust can use this penetrating power to stab a victim to death, or at least pin them to the wall. In the manga, Lust confronted Dr. Marcoh and threatened to use her Ultimate Spear to kill an innocent bystander, as a way to coerce Dr. Marcoh into following her back to Central, to Father.
Like her brothers, Lust has a Philosopher's Stone, embedded in her chest. She can rapidly regenerate any bodily wounds so long as the Stone has power left. Bullets are merely inconvenient to her, and dismemberment's just a temporary issue.
Lust's Story
Lust was the one who finished off Father Cornello, and she was also the one to coerce Dr. Marcoh in the manga version of the story. She ambushed Maes Hughes in a military compound and wounded him, but Maes slowed her down with a knife to the forehead, fleeing the building. Out there, Envy caught up to Maes and finished him off with a gunshot, right there in the phone booth. Lust was also tracking Colonel Roy Mustang and Lieutenant Jean Havoc, both of whom were becoming problematic for Father.
To take care of the issue, Lust dressed in charming human clothes, gave herself an alias and tried to seduce Jean Havoc to find out what he knew about Father's plans. Fortunately for the good guys, Jean never actually told Lust anything important, and during a mission, Roy and Jean both faced Lust. She was a villain, and Roy and Jean opened fire. It was no good; Lust regenerated all her wounds, and she stabbed Jean with her Ultimate Spear (this ended up paralyzing Jean from the waist down). Lust also dealt a mortal wound to Roy Mustang and shredded his alchemy glove, then left him to bleed out and die.
Next, Lust confronted Riza Hawkeye and Alphonse Elric, toying with them until Roy showed up. He had carved a transmutation circle into his skin and used it to blast Lust with repeated flames until she finally died for good. As she crumbled to ash, Lust noted the light of ambition and vision in Roy's eyes, and cruelly hoped that such vision would become clouded with fear, despair and horror later on. She was proven right when Roy was forced to give up his eyesight to open the Portal of Truth to save Riza Hawkeye's life. Even in death, Lust was still haunting Roy Mustang.