Callis Fel-Fotch Heppen-Bar Slitheen was a member of the Slitheen family in the 26th century. She broke into Justicia to rescue her nephews, Ecktosca Fel Fotch Heppen-Bar Slitheen and Dram Fel Fotch Heppen-Bar Slitheen. When their distant cousins and enemies the Blathereen tried to take over Justicia, they helped the Ninth Doctor to defeat them.
Callis and Ecktosca faked their deaths, and hid in the bodies of a Blathereen technician and the dead Blathereen leader Don Arco. They then reunited with Dram and planned to use the Blathereen gravity warp technology to kick-start the Slitheen family's return to crime. (PROSE: The Monsters Inside)