Jack, also known as "Subject Zero", is a notorious criminal whose crimes include piracy, kidnapping, vandalism and murder. She is also a biotic, possibly one of the most powerful human biotics alive, and considered so dangerous she was kept in stasis after she was caught. She is a potential romance interest for a male Shepard.
Jack is voiced by Courtenay Taylor.
Source: Mass Effect Wikia, "Jack," available under the CC-BY-SA License.