In the end, Lust decided, allegiance to That Person wasn't important after all.
Sure, That Person had raised her, and nurtured her out of the tortured form she'd been born in. Sure, That Person had promised her the Philosopher's Stone, and that someday she would become human, just like she always wished.
On the other hand, That Person was also extremely strict about dating.
You could attribute it to teenage rebellion, if you liked, or to homunculus-hormones, which could be quite fierce, or heck, maybe she was just living up to her name. But from the first time she saw Colonel Roy Mustang, the man she'd been sent to kill—well, suddenly all of That Person's promises and instructions became secondary.
The Colonel had been surprised to find her in his office. Very surprised, just as predicted. He didn't have time to reach for his gloves, or his run, or shout for his staff, as predicted. Only, instead of quickly running him through and then slicing his internal organs to ribbons, Lust found her pouncing on the man, pinning him against the wall, wrapping his collar around her hand, and kissing him thoroughly.
He struggled and flailed a little bit, but as the kiss grew more protracted he calmed down, slid his arms around her and kissed back, and that made it all worthwhile.
They had backed over to the couch, and Lust was just contemplating whether it was worth blowing her cover to slice his clothes off, or do things the slow way, when the office door swung open again without warning.
"Colonel, you—" the blond lieutenant started to say, and then stopped short at the sight before her. Lust growled, feeling homunculus kill-urges rising along with thwarted homunculus-hormones, and tightened her grip on Mustang's collar.
"Wait!" Mustang said, sounding more than a little panicked. He struggled a bit, again, but still couldn't match Lust's inhuman strength, and stilled. "I can explain this," he said to a point somewhere between the two women. "This isn't what it looks... glllck."
"I am aware of that, sir," Hawkeye said, slowly and ominously, stepping into the office and pulling the door shut behind her. She drew her gun, and warningly, Lust held up one hand and flashed out her fingers. Mustang yelped and went very pale all of a sudden, but Hawkeye ignored the noise. "I had just received the news that one of the homunculi had been sent to assassinate you, sir."
Lust pouted, and hugged Mustang possessively against her chest. "I wasn't going to hurt him," she said defensively. "He's too cute to waste like that. Come back in fifteen minutes and then we can fight, how does that sound?"
"If you think I am leaving him alone with you, you are very much mistaken," Hawkeye snapped.
Lust raised her hand, preparing to slice the intruder into ribbons, but there was suddenly a hard hand at her throat. It didn't bother her particularly, but she looked out of the corner of her eye at Mustang, who was now glaring very fiercely. "Hurt her, and you will have to kill me too," he growled. "—By which I mean, don't do it!" he added hastily, eyes opening wide. "I'm too young and handsome to die, even at the hands of a beautiful la... errmmm.. homunculus such as yourself."
Lust sighed, and tossed her hair back over her shoulder. "Oh, very well," she said. "You can stay, then. I'll even play nicely and share."
Homunculus-hormones, when roused, were demanding and unpredictable things. Which was probably how Lust found herself kissing the blond Lieutenant, as well. Not that she objected, in the end. Hawkeye didn't object, either. And Roy Mustang, finding himself stuck at the mercy of two of the most dangerously beautiful women in all of Central, certainly didn't object.
Homunculus-pheremones were even more intimidating than homunculus-hormones.