49 years ago - David Cain is born.
32 years ago - 17-year-old David joins the marines.
29 years ago - 20-year-old David starts working in covert ops.
28 years ago - 21-year-old David becomes a black ops assassin.
23 years ago - 26-year-old David abandons military service after Team 7 is shut down and becomes a freelance assassin, immediately attracting the attention of the League of Assassins.
21 years ago - 28-year-old David briefly trains a young Bruce Wayne, developing his theories about replacing language with movement. He is recruited into the League of Assassins.
19 years ago - 30-year-old David is chosen by Ra's Al Ghul to father & train "the one who is all".
18 years ago - 31-year-old David chooses Sandra Wu-san as the mother of "the one who is all". His daughter Cassandra Cain is born. he devotes himself and his life to her training.
3 years ago - 46-year-old David takes Cassandra Cain on her first assassination, but she immediately runs away after her kill. He battles Bruce Wayne to get her back, but retreats after she says her first work, "stop".
2 years ago - 47-year-old David is hired by Lex Luthor to frame Bruce Wayne for murder. He turns himself in after he is defeated by Cassandra Cain. The League sends assassins to kill him, but he defeats them all and goes underground.
David Cain is designed to be a supporting character. He was built to facilitate the backstory of Cassandra Cain, by far one of the coolest members of the Batman mythology. He was simply an assassin that had trained Cassandra, raised her without language and instead focusing her entirely on movement and martial arts. It's a cool idea, and David was a threatening enough presence simply as the figure that had facilitated such a heinous act of child abuse.
Of course, Cassandra was the first Batgirl to ever headline her own ongoing series, and it managed to spend over a decade exploring the character and all the ins and outs of her history. We discover that he was her father, that her mother is actually Lady Shiva, and that David Cain implemented his training at the behest of the League of Assassins. David became a more complex character as we explored Cassandra, and he becomes someone worth exploring as we structure out timeline.
Immediately, he seemed a perfect character to include in our covert ops unit, Team 7. He clearly has a military background before he entered the world of shadow assassinations, and this is the perfect place for that to happen. It also raises some very interesting ideas, because he's working right alongside Deathstroke, who is recognized as the greatest assassin in the world. What then is Cain? He walked the same path as Slade Wilson, but rather than continuing to work as a world-class assassin, he becomes something else. He devotes himself to his daughter's training (albeit, a particularly cruel and abusive form of training)
Deathstroke and David Cain are both parents. They are both regularly motivated by their children, but while Slade Wilson seems to play lip-service to his devotion to his children and will also just as often abandon them... Cain does not. Even after his daughter escapes him, once she earns the right to be free of hi, he respects it, even going on the run from the League of Assassins.
The question of who Cain is and where he stands within the pantheon of Batman villains is a fascinating one. I'd love to see more from this character.