Lily Kane is the little sister of Cleapatra and Oceana. She is sort of the "balance" between the powers of light and darkness; and somehow gained both, throughout sneakily drinking from the Eldar Witch's cauldron. She, of course, thought it was harmless water, as at the time she did not know of her stepmother's identity.
It took a lot of Clea's healing power to keep tiny Lily alive, and a bit of Oceana's too, as this was a new poison invented by the Eldar Witch in hope of coercing Clea to turn to the Dark Ways. Lily didn't know this, and became the first victim, nearly, apart from the house poltergeist.
She is called the "Yin-Yan Warrior" by her sister Oceana, who was the first to use this nickname, hence the fact that she is known as the "inventor" of this particular nickname adopted by Lily.