The Waif waited. Waited until the chores for the House of Black and White were done for the day. Waited until the last person who had come to the House to die was tended to. Waited until the last face was hung in the Hall of Faces. The Waif waited.
Night was the time the Waif struck, when everything seemed still in the House. Jaqen had retired and Arya had gone to bed. It was the perfect time for the Waif to strike.
The Waif found Arya asleep in her bed. She hiked up Arya's dress. She wet her right middle and ring fingers and stuck them inside Arya. She put her right hand on Arya's mouth.
This was what the Waif wanted. Arya's eyes growing bigger from the shock of the assault. Her inability to cry for help--after all, she was No One, and No One needed help. Arya getting uncontrollably wet as the Waif moved her fingers inside her. The way Arya couldn't control the thrashing in her bed. Arya shuddering as she was forced to come against her will.
As Arya recovered from the Waif's assault, the Waif smiled, as if to say she was in the House first. That the Waif was truly No One, and that Arya, in comparison, would still be Lady Stark, she of a noble Westeros home, she of a class far too dignified to ever be a Faceless Man. No matter what Arya did, she would never be like the Waif.
The Waif cleaned her fingers on Arya's dress and departed for her bed, leaving Arya wondering what had happened to her.