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 endangers 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.