Real Name: Jack Power
Occupation: Student (elementary school)
Identity: Secret
Legal Status: Citizen of the United States with no criminal record, still a minor
Place of Birth: Richmond, Virginia
Marital Status: Single
Known Relatives: James (father), Margaret (mother), Alex (brother), Julia, Katie (sisters), Roy Craig (grandfather)
Group Affiliation: Power Pack
Base of Operations: New York City
First Appearance: POWER PACK #1

History: Jack Power is the younger son of Dr. James Power, a physicist who was working for the United States government on a means of producing large quantities of energy cheaply through combining matter and antimatter. (The subatomic particles composing antimatter have opposite charges of those of the equivalent particles of matter. When matter contacts antimatter, both are converted entirely into vast quantities of energy)

An alien Kymellian named Aelfyre Whitemane (nicknamed "Whitey") learned of Dr. Power's discovery and intended to stop it from being used. Whitemane knew that a device such as Power's, although Power did not realize it, would trigger a chain reaction that would destroy the Earth. Moreover, he knew that Power's device was sought by the alien Z'nrx, nicknamed "Snarks," who intended to use it to conquer inhabited worlds.

Snarks captured Dr. Power and his wife Margaret, intended to force him to reveal the secret of his device. Whitemane saved the Powers' children, Alex, Julie, Jack, and Katie, from being captured, but he himself was fatally wounded in the process. Dying, Whitemane transferred a different one of his superhuman powers into each of the four Power children, and asked them to use their new abilities to save their world.

Jack received the power to alter the density of his body's mass, and therefore called himself Mass Master. The four children called themselves the Power Pack. Power Pack thwarted the Snarks, rescued their parents, and destroyed Dr. Power's dangerous antimatter converter, thereby saving the Earth from destruction. The four children have since continued to have adventures as Power Pack.

During his brother Alex's time with the New Warriors, he "borrowed" all his siblings' powers against their wishes. And yet during a New Warriors' mission, where the Smartship Friday was nearly sacrificed in order to stop the Dire Wraith Queen Volx and the Neo-Neutralizer weapon almost cancelled all of the Warriors' powers, Alex inadvertently returned the powers back to the Power Pack.

The Power Family have recently moved to Bainbridge Island, approximately ten miles away from the city of Seattle, Washington.

Height: 4 ft. 5 in.
Weight: 66 lbs.
Eyes: Blue
Hair: Brown

Strength Level: Mass Master possesses the normal human strength of a boy of his age, height, and build, who engages in a normal amount of exercise for his age (mostly through playing).

Known Superhuman Powers: Mass Master can alter the level of density of the mass of his body through an act of will. His control over his density does not operate in the same manner as the Vision's similar power. Unlike the Vision, Mass Master cannot become intangible or diamond-hard, nor can he increase his weight beyond its normal level. Moreover, the Vision retains his normal size throughout all his density changes, whereas Mass Master does not.

Instead, by mentally expanding the amount of space between the molecules of his body, Mass Master takes on gaseous form. His consciousness remains intact when he is in this form and continues to control his body in gaseous form. Mass Master can expand his size to gigantic proportions. He can also float and fly in the air when he is in gaseous form. Apparently Mass Master does not need to ride air currents to move through the air, but it is unclear exactly how he propels himself forward. In his gaseous form solid objects pass through him harmlessly; however, he cannot carry solid objects when he is in this form. Mass Master's gaseous form is cloudlike, and he can make himself resemble a dense fog so as to conceal his teammates. Mass Master usually maintains his normal human shape (at least or the upper portion of his body) when he is in gaseous form, but he can alter this form's appearance. He can also maintain control over his entire gaseous substance if portions of it become separated from him.

By mentally reducing the amount of space between his body's molecules, Mass Master can shrink in size to a height of only several inches. Mass Master thus compressed this full mass into a very small volume. His body is harder and thus more resistant to injury when he is at this size. At this size, by jumping atop an adversary, Mass Master creates his "jackhammer" effect: the adversary feels the entire weight of a young boy hit him atop a very small area of his body, and the sudden impact usually knocks down the victim.

Paraphernalia: Mass Master wears a special costume created by the Kymellian smartship Friday from unstable molecules. The costume shrinks and expands in size as Mass Master does, and turns gaseous and solid when he does.

The costume usually occupies another dimension, but Mass Master can mentally command it to materialize on his body in this dimension, and simultaneously shift his normal clothing to the other dimension. He can also mentally reverse this process.