Jack Harkness' mother
Talk about it here.
Following the attack by the "worst creatures in existence," she and Javic were the only survivors in their family; Gray was captured after Javic accidentally let go of his hand, and Franklin was killed by the creatures, with Javic returning home to find his mother cradling Franklin's corpse. (TV: Adam)
Javic, then going by "Jack Harkness", later implied in a conversation with Esther Drummond that he did not know what had happened to his mother, suggesting that she was still alive when he left his time frame. (TV: The Categories of Life)