Brash and ornery, Jack Power was the "wild child" of the Power siblings. He frequently insulted and argued, both with his teammates and his adversaries. Queen Maraud, Power Pack's archenemy, feared him more than the other children because she thought he would be the most likely to take advantage of the lethal nature of his powers.
However, his bold nature also made him the main fighter on the team. He possessed a tremendous amount of loyalty and courage and usually kept a cool head even during stressful situations. Jack jumped into the East River to help his sister Katie escape from an ocean liner, and even taunted the Snark Jakal who was about to kill him with "Can't even kill someone until Mommy tells you to?!"
Jack was perhaps the most innovative member of the Pack in terms of developing new power variations. When he received new powers, he was able to figure them out quickly and invent new uses for them. He was significantly better at controlling his gravity powers than Alex.
Jack retained his bold personality, and continued to develop new applications of his powers, including a "density field," but he also showed a kinder, gentler side. Jack opted to leave the team to care for his mother.
At some point, Mass Master briefly rejoined the Power Pack and teamed up with Devil Dinosaur, Gwenpool, and Moon Girl.
