I didn't expect Yuno to have such a past. I assume she was serving the Sumeragi family from the beginning. To be honest, I'm not sure if she really needed this kind of background. It's not like the Sumeragi couldn't have trained her as well. Though that being said, the Sumeragi aren't really the kind who would employ assassins. But spies/investigators are just fine. That's what I believed Yuno to be, anyway.