Bio: Jennifer Walters is a scientist employed by S.H.I.E.L.D.
She was able to take the original serum that turned Bruce Banner into the Hulk (Ultimate) and tweak it's properties enough to allow the subject injected with the serum to possess all the "Hulk" traits, with none of the anger associated with it.
Seeing what Walters was able to accomplish, Betty Ross, knowing that Nick Fury had sent Wolverine to kill the Hulk, convinced Nick Fury that Walters was committing an act of treason by selling the serum to the Chinese, and while Fury and S.H.I.E.L.D. were tracking and taking down Walters, Ross took the serum, made her way to Hulk and Wolverine, and injected herself with the serum to become She-Hulk (Ultimate), all in an effort to save the Hulk.
Notes: The Ultimate version of this character is not an alternate identity of the regular Marvel character as the Ultimate universe is a distinct reality.
See here for more information.