Sasha Martin had a trouble childhood after her parents died, in her teens she found a piece of alien technology and saw it as her chance to do whatever she wanted. She took the name Southpaw and assembled her own gang called D-Generation using her powers to do whatever she wanted, and take anything she wanted whenever she wanted. Eventually, while solo, she was apprehended by the New Warriors and sent to the Scott Lang Memorial Prison aka "The Big House" where superhuman inmates were shrunk with Pym Particle gas to be more manageable.
The Mad Thinker needed Southpaw's assistance to engineer a breakout of The Big House, knowing that her grandfather Holden Holliway would ensure that she was remanded to his custody and re-enlarged. When the escapees endangered Holden's life she helped apprehend them and was then remanded to the custody of She-Hulk, possibly the only person strong enough to keep her in line. When an Infinity Gem-empowered Titania fought She-Hulk, Southpaw took the opportunity to flee, though the example She-Hulk set for her might have made her rethink her criminal impulses.
Powers & Abilities
Southpaw possesses an alien gauntlet that grants her superhuman powers. It grants her superhuman strength and the ability to generate an energy projection in the shape of a hand. She experiences psychic feedback when enough physical force is used against the energy hand.
In one possible future Southpaw had become New York's preeminent criminal lawyer and defended She-Hulk against the Time Variance Authority who wished to wipe her from continuity for attempting to change the past. She also has a sidekick named Kid Glove who has the right-hand gauntlet.
The name 'Southpaw' refers to a person who is left handed since her gauntlet was made for, and is worn on the left hand.
The gauntlet she wears is the left equivalent to the right handed gauntlet worn by Joseph Green, the hero drill instructor called Gauntlet.