Sistask is the product of a terran experiment, courtesy of mixing terran and zerg genes to form mutates to fight against the zerg. Sistask, like many of her kind, fights willingly against the zerg.
In terms of appearance and anatomy, Sistask is mostly human, though there are enough differences to make those around her uneasy. Despite this, Sistask has found a home among the marines of the Alliance, their violent personalities being similar to her own. Still, many of Sistask's violent impulses must be held in check by anti-zerg nanites.