The Jackers are a group of inhabitants of Nia (Sunny, Sandy, Angellene, and after the raid at the laboratory, Pippin) intent on stealing and making use of Project Kokiro, enabling them to travel to the past. The origin of their name is debatable. It would seem logical that their mission involves "hijacking" Project Kokiro, but Sunny has an alternative etymology: "[Give us Kokiro] or we'll jack you up. Right?"
Although Angellene manages to infiltrate the laboratory disguised as a caretaker, the Jackers are not always a subtle organisation; indeed, Sunny and Sandy have been seen in tee-shirts that boldly state the Jackers' name on the front. They do, however, carry wrist-strap devices that seem to enable a change of clothes at any moment.
It is as yet unclear what the Jackers' purpose is in travelling to the past, and Sunny at least does not appear to know it herself. However, Angellene seems to believe that the mission is of such life-changing significance that Sunny and Sandy will not "be able to go home after this".