The prototype for Jack was developed in Russia. Since it was just a step in the production, the skin was removed from the mecha. Prototype Jack (or P. Jack, for short) was built only for power, so his power capacity far surpasses that of the completed Jack. The tournament's sponsor, Heihachi Mishima, has entered P. Jack into the contest in order to face him against Jack.
After the first Rave War, the remains of the first Prototype Jack unit (a first-stage model of the final Jack model) were almost destroyed by Jack's rampaging combat abilities. Upon reactivation, P. Jack complained that his alter-ego had received a refit, and he repeated his plea until his fuel ran out. Now resting in Kazuya Mishima's laboratory, P. Jack was left untouched for months before a back-up fuel supply reactivated him, and once again he began his constant pleas for a new look. To quiet him, Kazuya gave his captured scientist, Doctor Bosconovitch, the task of remodeling the robot.
With a cunning memory change, the doctor managed to convince P. Jack that his new body afforded him superior protection to Jack's, despite it being only a hat and sunglasses. Kazuya is currently goading P. Jack to demand more programmed maneuvers in time for The King of Iron Fist Tournament 2. Having repeated his fighting moves over and over (to such an extent that the doctor disconnected P. Jack's voice box), P. Jack now feels his next encounter with Jack will have a much different conclusion.
Ending Description: Prototype Jack is traversing inside a tunnel and turns his leg booster on in a vertical opening. He comes out of the shadows and flies straight up into the sky. As he flies away, he explodes and three screws have fallen out onto the ground.
In Tekken: The Motion Picture, three Prototype Jack robots are programmed to protect the labs in which Lee Chaolan and Heihachi Mishima had their test subjects hidden. There comes a point in time where the three Prototype Jacks and Jack-2, along with Lei Wulong, have a battle in the middle of the lab.
Prototype Jack has appeared in Tekken Tag Tournament as a playable character.
Prototype Jack appeared in Namco X Capcom as an enemy character. He first appears in Chapter 14, along with a second Prototype Jack, as part of Waya Hime's army when she attempts to ambush Bravoman and Jin Kazama.
Prototype Jack appeared as a playable character in Tekken Tag Tournament 2, sporting the same look he received in the original Tekken 2 and Tekken Tag Tournament. He plays the same like Jack-6 while offering different moves that Jack-6 doesn't have.
Ending Description: A large Prototype Jack is being built, then comes along a Prototype Jack walking forward to it and then stares at it while the body of the stomach is opening. It enters into the stomach of the body and sits down, while pressing the button which activates the giant P. Jack to get ready to launch. He gets ready to launch with a countdown, while the other construction Prototype Jacks leave the perimeter. Just as the giant P. Jack launches, a construction Prototype Jack rushes in carrying a large screw, desperately waving to the pilot in an attempt to get it to abort the (incomplete) giant P. Jack's launch. However, the pilot misinterprets the worker's actions as a simple good-bye wave, and does a thumbs up. The construction Prototype Jacks are left to helplessly watch the giant P. Jack's launch. It then explodes high in the air, with three screws falling back into the hangar.
All Jack robots embodies the huge, slow and powerful fighter archetype: they need a few solid hits to destroy an opponent, but they're very slow and their limited moveset can get predictable, so they require a good timing. Jacks have good damage, good pokes, and good throws, but the real thing that makes them shine is their ridiculous range on most of their moves. Most of their punishers hits from very far away.
In Tekken Tag Tournament 2, Prototype Jack returned with a moveset nearly identical to Jack-6's. There are some differences between the two, some of Prototype Jack's moves are different and, unlike Jack-6, his arms cannot be extended, but their gameplay is essentially the same.
While other characters do fight, what Jack robots do is best described as "using brute force to annihilate the opponent", and their fighting style is listed as "Brute Force" accordingly. Prototype Jack is no exception.
- Prototype Jack is the only Jack model who doesn't have plutonium blood (his is gasoline).
- P. Jack is also the only Jack robot that has competed in more than one tournament in the Tekken universe, despite the fact that he has a large difference between his Tekken and Tekken 2 appearance.
- Prototype Jack was destroyed by Jack-2 in the second tournament, as examined in his Tekken 2 ending.
- Compared to the other Jack models, Protoype Jack is added with a rocket booster built in its leg. P. Jack's internal support system can provide up to three "detonations" to the booster. Its arm is also somewhat "loose" as it can spin its hand freely without wrist limitation.
- In Tekken Tag Tournament 2, If the player leaves Prototype Jack alone for about six seconds, he will shut down and reboot himself.
- Prototype Jack's Tekken Tag Tournament 2 ending is a re-imagining of his Tekken 2 ending.
- Prototype Jack, along with Jack-2, Gun Jack and Jack-6 are the only Jack robots to appear in more than one game.