Cassandra Cain is many things.
Born and raised by a man who is considered one of the worst human beings on the planet by anyone who has heard the tale of her childhood, she was expected to be a world-class assassin and living weapon by this age. The bodyguard of Ra's Al Ghul and the crown jewel of The League of Assassins.
Cassandra Cain is not that.
At age 18, Cassandra Cain was reborn by choice, born into a new world and into her new destiny. The Silent Knight. The Darkness. Batgirl. Or at least that's what she thought her destiny was. Cassandra spent her next few years carrying on the legacy of the title that her one of her best friends invented. Cassandra did incredibly well, more powerful then her predecessor, stronger then her peers. Cassandra had become somewhat of a crown jewel of her new home. Her new family. Then it all went wrong.
Cassandra was drugged by her father and Deathstroke and turned evil, leading the group that she had spent so long trying not to become one of, Cassandra nearly killed her pseudo brother Tim in this drugged state, one of the few people who she knew believed in her, losing her family. In revenge, she goes against her oath not to kill and tries to kill her father and Slade Wilson, having to be stopped by her pseudo older brother Dick before she could commit the act. Cassandra is left broken but commits to making amends joining a team at Batman's request and fighting hard to fix her reputation. But then just when she thinks she can escape. That is when her father reappears, driven by rage, she takes him down but is stopped from killing by Batman who adopts her and makes her officially Cassandra Wayne.
Cassandra is fine until he is also taken from her. In his will, it states that she must leave Gotham in case of his death, she obliges but gives her mantle to the one person she has ever loved and trusted fully. Stephanie. Oh Stephanie. From day one, she has loved her and vice versa. She gives the Batgirl name to Stephanie and leaves the love of her life behind to move to Hong Kong and become the Black Bat.
Her first mantle without a previous holder and yet it is empty, without her family she feels alone and empty, fighting crime until she can be her own established hero while in Gotham, her older brother becomes The Batman, raising an assassin's child to be better much as Bruce had done for Cassandra. She stayed there until her younger brother Tim, now known as Red Robin brings her back to Gotham, and at that moment she is whole again.
Dick steps down as Batman when Bruce returns from the dead, despite offers to stay, Cassandra stays away from the Batgirl mantle but then she is offered something.
Bruce is getting old and while he wants to be Batman he knows there are nights where he cannot be Batman especially when he is raising his son and newborn daughter Helena. Cassandra is given the mantle she has been training for all her life without knowing. On the nights where Bruce cannot fight.
Cassandra Wayne is The Batman