DC Comics: Cassandra Cain (Black Bat)

The daughter of two notorious assassins, David Cain and Lady Shiva, Cassandra left her father as a child after she killed a man and saw his pain and fear while he died. After saving Commissioner Gordon's life, she became the new Batgirl with Batman and Oracle's blessing, and was later adopted by Bruce Wayne. Later still, she gave the role of Batgirl to her friend Stephanie Brown, and became Black Bat.