Lust was desire. Greed was wanting more of desire. He was a benefit for her and made the job easier. He was very convincing, but then again, most illusions were. They tricked you into believing they were real. But Lust wasn't a real girl, Greed wasn't a real boy. They worked together because they'd destroy anybody who wasn't a mirror image, only your counterpart knew how to handle you. Even as a sin, they were the only ones who were pretty much incapable of draining the other.
She wrapped her legs around his waist and let him take control. She kissed him and the more he turned her on, the more she desired him, the more she desired him, the more he'd want her and it was a cycle with them, they fueled the other. But despite them 'making up' every time Lust saw one of the marks on Greed's body it pissed her off so she'd duplicate it. She'd bite just as hard in the same spot, claw him just as hard so the marks were still there and tender again but this time, they were hers. She threw in a few new ones too but she was Lust and he was Greed and for all the damage she did to him, she made sure to be equally soft and soothing. She liked to be luscious with him, they always had time.. why not fully utilize it.