Harley and Joker, forced a woman to be their surrogate and took the baby many years ago. They raised the child to be just as insane as them, their morals being murky. During one of the crimes the three of them were doing, Bruce grasped the young kids arm thinking they was one of the victims of the crime, only for Harley to yell at him

"Hands off my kid bats" she spat. Leaving Bruce and the other members of the bat family shocked and confused.

"Your.. what?.."