Okay, the simple problem with introducing Ryuuko is that the bio establishes she's technically in the care of the police right now. And because she's new to all this, she's got quite a malleable personality and [i]won't[/i] run away (because her non-violent blank slateness is rather the point) to spontaneously do stuff.

Which means the question is "what people are actually responsible enough and in a semi-public location?"