The Cupid of Crime
Harleen Quinzel was born in the Gotham City Hospital. She was a normal child, by the way she acted as a normal kid, you wouldn't guess that she would be one of the most insane people that Gotham has ever seen. In High School, she was on the gymnastics team. Harleen had tried out for the cheer leading team, but had been quickly rejected by the squad because they thought of her as a strange and skinny girl.
She graduated High School with a gymnastic scholarship and went to Gotham University to study psychology and make friends she never had. At college, Harleen found out that psychology was to complicated and she couldn't get the grades she wanted to graduate college. So desperate, Harleen seduced her professor and passed Psychology and graduated to take a job at the Arkham Asylum for the Criminally Insane.
It was there where she met the Joker and understood her true potential as a person. When he broke out, Harleen was devastated and wished for him to come back and talk to her, tell her more about his painful childhood woes. She found his tale soothing and dark, and she couldn't wait to hear more of it. It was then when Batman brought the Joker back to the asylum that Harleen snapped.
She then became Harley Quinn and broke the Joker out of the asylum and he brought her into a world of crime. In that world of crime, she has made friends with Poison Ivy, her one true friend which she found when she saved her one night, and made an enemy in Batman because he always tries to hurt her 'Puddin'.