Himself.

He couldn't bring himself to try it on Ultrafuta, even if she was no longer really his mother, even if she was a better test subject as a futa who would produce more cum, from a larger cock (though not much larger, given Ultraboy's length and girth). Besides, the show she had put on, sucking, jerking and fingering herself had made him aroused as hell and he wasn't a hundred percent sure he could control himself in her presence, not if she made the same set of advances on him she had before and he wasn't ready to go there. Or at least, not because he lost control of himself. And he needed to get off and wanted to be more in control of himself at his next encounter with Nishi. He locked the door to the equipment room (it was in the basement of their large house).

The milker was made of a flexible material. It looked like an incredibly thick condom with a tube emerging from the tip and going around to a canister, capable of holding a substantial volume of semen. Despite being much bigger than the Semen-Lord, the material easily stretched, covering his cock from head to balls and he pressed the button on the canister, activating the suction. The milker sealed at the base of his cock and tickled the tip of his slowly hardening cock. He pushed a second button on the canister and activated the rest of its systems.

Vibrators and motors activated and the milker began working, small amounts of lube leaking out of holes in it, lubing his cock as it rippled against his cock in a fashion against his cock, almost like living tissue. And the vibes began, pressing against his glans, teasing nerve clusters, massaging his cock to hard stiffness in less than a minute.

It kept going remorselessly bringing him off with a speed that he wouldn't have thought possible, given how well he'd lasted with Nishi, a beautiful futa, if under...odd circumstances. But there was nothing human in it, no hesitation, no mercy, no exhaustion, merely endless, robotic pleasure, placed where it would get him off most efficiently. Less than two minutes later he came again. Only a small amount this time, having not had time to build up a new load, it hadn't been long whether measuring from time of arousal, or time since he'd last cum. When his eyes uncrossed, he turned off the motors (ending the pleasure/pain of it massaging his softening cock), then the suction and pulled the milker off himself. The vacuum seal broke with a pop and, as he put it down, it began its self-cleaning, vacuuming every drop of semen into the canister. He examined his softening fuckstick as closely as he could, even running a black-light over it. There was no cum leakage. It was perfect.

He heard something upstairs, stuffed his softening cock back into his pants and headed upstairs.