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Yuffie.
Surveillance: close observation of a person or group, especially one under suspicion.
Yuffie watched Rinoa constantly.
There were a number of reasons for this:
A) What made this woman so special?
B) How had she come back from the dead?
And C) Did she know anything about Tifa?
There were other questions too, but these were the main ones.
So Yuffie watched.
Of course, it was only a matter of time before Rinoa noticed and being the woman that she was, she confronted Yuffie.
Yuffie, being Yuffie, did not ask her questions out right, but sought to trick Rinoa into revealing the information that she sought.
Ten minutes later and all Yuffie had to show for her effort was a grudging respect for the pretty brunette and perhaps a partial answer to A).
Rinoa was not the pretty, flighty, butterfly she first appeared.
So Yuffie was blunt.
Rinoa didn't know what made her special, she was what she was.
She had no real idea of how she'd come back from limbo (as she put it) but suspected it had something to do with Squall believing in her return.
And yes she had known Tifa.
She was no more forthcoming than that, saying only, that she doubted Tifa would be left behind again.
With that Yuffie had to be satisfied.
So she stopped watching Rinoa and took to watching the square, hoping against hope, that one day a brunette would suddenly appear.
Lightning had been known to strike twice after all.