Bellatrix enjoyed spending her father's money almost as much as she enjoyed torturing and sometimes killing other people. Unfortunately she was dressed in one of her favorite dresses that day and therefore was unable to detach somebody's body parts without running the risk of making a stain on the black silk.
Behind her trolled a young man who hadn't said a word since they had arrived at Diagon Alley. His arms were loaded with packages, and yet he still managed to open the door for Bella as they reached Madame Malkin's. "You're going to drop it all," she hissed at him. "Take those things home, put them away, then come back for whatever else I find."
Once he was gone Bellatrix busied herself with the items displayed on the counter while she waited for somebody to come. It was then that she noticed Antonin standing there, however his name was momentarily escaping her. Luckily Bella had mastered the art of pretending she was completely oblivious to everyone around her, which then forced them to notice her and in turn, gave her more time to figure out their name. Unless of course she was lucky and they said their name for her.
She grabbed a pamphlet for custom cloak pins from the counter and busied herself with reading it.