A fan-favorite, and all around epic character, Cassandra Cain is one of the many who took on the mantle of Batgirl. But her origin was far from Batman-esque. She is the daughter of legendary assassins David Cain and Lady Shiva (or at least in her original origin she was), where she was raised to be the world's greatest assassin. But, because of who her parents was, they didn't raise her in a loving and kind way, and was held back from talking from others. So thus she bascially became a mute.
She was taken in by Batman though, and was raised in his style of crimefighting. Through this, she became a true Batgirl and member of the Batfamily. She's worked wtih Oracle, Tim Drake, and many others throughout her time.
In the New 52 universe, she goes by the name Orphan (as Barbara is still Batgirl currently), and has recently joined the team.