It is implied that Kara was Mordred's childhood sweetheart at some point, though exactly when, at what age, is unknown.
It's also most likely that Kara's family (or at least close friends) were killed by Uther, as her hatred of him and Arthur is deep-set.
Following years of running, due to the persecution of those with magic, Kara was set in her beliefs, to the point of being blinded by them. She rejected all Arthur's attempts at change, seeing him as no better than his father, and accusing him of murder. She claimed to fight for freedom, and the right to be who she was. Despite this Kara was also dark and possibly delusional, and had no problems with murdering in cold blood, seeing all connected to Camelot as evil. Her hatred of Arthur was so strong she rejected her chance of freedom, claiming to have committed no crime, despite being on trial for murder and not for magic.
However she also had a nicer side, and loved Mordred, to the point where she refused to betray him to Arthur.
- Nimueh used this name (same pronunciation but different spelling with a "C" instead of a "K") when she infiltrate Camelot in the episode "The Poisoned Chalice".
- Some fans speculated early on that Mordred and Kara's relationship would be similar to that of Merlin and Freya's in The Lady of the Lake, but the only real likeness between the two are that Freya and Kara were both Druid girls who were kept hidden in Camelot by somebody close to Arthur. Whereas Freya was only a killer because of her curse (and her "crime" was only that she was a Druid), Kara kills willingly for hatred of Arthur and Camelot (her crime was killing several men, whom she claimed were merely casualties of war, and for attempting to stab Arthur after swearing she meant him no harm).
- Kara's death serves as Mordred's motivation to kill Arthur. As well as motivation to tell Merlin's secret to Morgana.