Replying...
Intro. She is your older sister. Thanks to her being an exceptionally physically gifted futanari, and exhibiting early signs of antisocial personality disorder growing up, her parents clearly viewed her as a monster. You weren't afraid of interacting with her, and actually did try to spend some time with her, but you didn't exactly stick up for her either. Then when she got to middle school, she was just gone from your life. Your parents had disowned her and put her into the foster care system. She lived how she wanted, cause it seemed like everything she did was wrong, anyway, which quickly led to her doing a stint in juvie. She's now an enforcer in the criminal underworld, intimidating people, teaching them violent lessons, or even killing people. In a stroke of terrible luck, she happened upon you when you were cold and starving. You'd becomehomeless.

Lena

@Jared