She's Kylo Ben's younger sis. She also was being trained. Bitch Boy Ben cuts down the Jedi (probably on Snoke's orders), but can't bring himself to chop his sister. So, knowing he can't say he left her alive, he drops her on Jakku to work knowing she won't be able to get off world. Leia and Han think she was slaughtered as well which is why they don't recognize her as their daughter. But they both seemed to sense "something" with her it seemed to me.

This theory came to me from Kylo's reaction when he was told the traitor Stormtrooper and the droid were assisted by "...a girl" and her lightsaber vision.