Jack

"Turns out, mess with someone's head enough and you can turn a scared kid into an all powerful bitch."

Subject Zero is a human vanguard and confirmed squad member. A young woman with a mysterious past, she was once the subject of experiments that resulted in a dramatic enhancement of her biotic abilities; which also caused her unstable and rather fiery personality. She is covered from head to toe in tattoos, marking kills and events in her life. She has run with various gangs and at one point was a cult member, keeping her head shaved after her departure. She is described by Shepherd as "dangerous, powerful and extremely violent." Her character trailer heavily implies she will have a romance subplot with Commander Shepard.

She is the third new party member to be revealed for Mass Effect 2. Her character trailer was first shown at Tokyo Game Show '09.

Source
Character Trailer