Michael
The immortal allowed a moment to appreciate the girl's expression and the little cries of pain, before starting to set up one more. When Sakura finally got across to the other side of the rope, Michael was waiting for her with a pair of curious devices.
"Congratulations," Michael said, taping the leads to the sides of her folds, "Now, I bet that tired you out a little bit, so why don't we get you energized again."
With that, the immortal turned up the dial, sending a painful, though not particularly harmful, current through the leads, setting it to cycle on and off at regular intervals, giving her short breaks between each bout of current.
"Ah, I'm sorry," Michael noted as he watched her reaction, "That's probably not enough."
With that, he fired up the other device, a cattle prod, and pushed it into one of her breasts, then the other, switching between them seemingly at random.