Simple:
Selina Kyle, born to a ****ish mother and a father who seemed to hate her, ran away from home at the age of thirteen. To survive, she became a thief, later also becoming a prostitude out of nececity. Eventually ditching the life of ****ing for bigger scores, she became a high profile cat thief known as Catwoman, and used her ill gotten gains to make her way into Gotham's high society, possing as a socialite who simple made some very good investments. Soon, her thieving took on a Robin Hood like status as she saw the decadence of Gotham's East End in comparison to her own wealth. Her ally through all of this is Holly, another, younger prostitute who Selina befriended and is now her confidant. At some point, she discovers that the reason her father hated her is because her mother had cheated on him with crime boss Carmine Falcone, and she was his illigitimate child.