The original Jack was built by the Soviet Union in order to combat Kazuya's plans of world domination. With each itineration of the series, Jack is replaced by an upgraded version, with Prototype Jack being the only model to compete in multiple tournaments.
Similar to Kuma and Ganryu, Jack uses his arms to deliver crushing physical attacks. These moves often show his arms being utilized as drills, guns, and jackhammers. He has few kick attacks, mainly using his legs for powerful stomp attacks on grounded opponents. Due to Ganryu's absense in Tekken 3, Gun Jack in that game adopted some of the former's moves. His next iteration, Jack-5 in Tekken 5, lost them.
The Jack-6 model in Tekken 6 can be customised to resemble previous models, including Prototype Jack with the 'PJ6' series of items.
Jack: Appeared in the first Tekken. It was produced by the Soviet Union in order to stop Kazuya Mishima's plans for world domination.
Jack 2: Is the upgrade of the first Jack model and rivals Prototype Jack.
Prototype Jack: Prototype Jack (or P.Jack) is the rival of Jack in Tekken 1 and 2. It is also the first Jack robot given the ability to fly. Heihachi Mishima employed him in Tekken in order to see who is more powerful: him or Jack. He was defeated by Jack 2 in Tekken 2.
Gun Jack: Gun jack is the evolution of Jack who first appears in Tekken 3. He was remodeled by Jane as Jack-2 was destroyed saving Jane in Tekken 2. Besides Ling Xiaoyu, he is the only character with two different endings. Canonically Gun Jack was destroyed by the Tekken force.
Jack 4: Jack-4 is an unplayable character in the Tekken series. It was only mass produced to attack Heihachi Mishima and Kazuya MIshima before the events of Tekken 5. There is no Jack robot that participated in Tekken 4.
Jack 5: Jane was recruited by the robotics division of G-Corporation and created Jack 4. Through the data of Jack 4 Jane created Jack 5. She employs Jack 5 in Tekken 5 to test her creation.
Jack 6: Kazuya Mishima ordered his own creation of Jacks in Tekken 6 to destroy the Mishima Zaibatsu. Thus the creation of Jack 6.