Lucy is the daughter of the Joker and Harley Quinn.
Injustice: Gods Among Us Year 2
Harley reveals to Black Canary that she has a four year old daughter named Lucy who is being raised by her sister. After discovering she was pregnant, Harley left the Joker for almost a year to have their baby. She decided to give her up because she knew the Joker would never want a daughter and bringing her into their life would be dangerous. When Harley returned to the Joker after being gone so long she thought it was a joke that he hadn't even noticed she was gone.