23 years ago - Holly Robinson is born to a runaway in Gotham's East End.
18 years ago - 5-year-old Holly is saved from a corrupt cop by her neighbor, Selina Kyle. She begins spending most of her time in Selena's apartment.
16 years ago - 7-year-old Holly's and her mother are given thousands of dollars by Selina, who tells them to take care of each other.
14 years ago - 9-year-old Holly's mother moves in with her drug dealer boyfriend. Their savings are soon completely gone.
12 years ago - 11-year-old Holly runs away from home.
9 years ago - 14-year-old Holly OD's, but survives, and starts fighting her addiction.
8 years ago - 15-year-old Holly breaks into the building she shared with Selina and finds her there. She becomes her roommate and confidant, regularly training with her.
5 years ago - 18-year-old Holly is kidnapped & beaten by Black Mask's men to torture Selina, but she fights back and escapes.
3 years ago - 20-year-old Holly becomes a new Catwoman when Selina has her daughter Helena, and regularly travels to be with her.
now - 23-year-old Holly runs afoul of Penguin's operation and is beaten so badly she is on life support. Selina returns to Gotham to become Catwoman again and go after Penguin.
Holly was actually introduced in the pages of Batman: Year One as an underage prostitute. I totally get the world they were trying to create, and sometimes when you're telling hard-boiled thriller stories, you create characters in really lousy situations in order to make your world feel gritty. I get it. Still, it's not a story that's particularly comfortable to read. That version of Holly went on to be killed by her husband in a story we could probably all do without.
After one of DC's many continuity resets, this time Zero Hour, Selina Kyle has history changed (thankfully) so that she was never a prostitute... but this change left Holly's status rather up in the air.
She was later included in the ongoing Catwoman series as a roomate and confidant, and suddenly we had a character that we could all totally get behind. She was spunky and fun, but with a twisted past, someone you could really derive fantastic stories from. She was just a great, great addition to the Catwoman mythos in particular and to DC in general.
She's had he share of really weird story lines; most notably her extended role in the ongoing series 52, which led to her being somehow empowered to fight New Gods on Apocalypse alongside Harley Quinn.... which... is definitely insane. Her absolute best story came when Selina stepped down from her role as Catwoman to give birth to her daughter. Holly was an obvious choice as her successor, and for a while there we got to enjoy a very cool, fresh take on Catwoman.
When we decided to have our Selina not only give birth to her daughter, but to be more a much more active parent (who knows who that daughter is going to grow up into), it followed very naturally that Holly would take over in our timeline. It was an easy decision to make, because Holly really does make a fantastic Catwoman. We created the story of Selina's return primarily because Selina is such an integral part of Bruce's story and she really does need to continue to fill the role as long as Bruce is Batman, but it is cool knowing that Holly is there and totally primed for a "heroic" career all her own.