1. Harley Quinn
Harley Quinn was the first element that was created in the animated universe and then later incorporated into the comics. This in and of itself shows her importance as she did what no other character had up until that point. Harley was not just interesting because she gave name and face to Joker's henchmen. Harley gave us perspective into the Joker that we had not had before. She was the only one to see him at his most vulnerable quiet moments. She comforted him when no one else would and saw the humanity in him, in spite of its sincerity or lack there of. Harley made us not only see the henchman in a different light but brought an entirely new element to the Joker as well forever adding to his character. Ultimately, a back story was created for Harley. She was once known as Harleen Quinzel - a doctor at Arkham Asylum who treated Joker before becoming obsessed with him. He seduced her and she eventually became his girlfriend. Her character has undergone many alterations over the years but they all stay true to the character created in the Timm/Dini Verse. The team essentially did it again when they changed Mr. Freeze from a hollow and pointless character to a complex and sympathetic villain with no emotions that was always trying to save his wife. Most shows only pull from the source material. These shows added to it.