to sum up fuu simply, she is a fourteen-year-old student and she's very smart and analytical (perhaps too much so), and she tends to be reserved and quiet, even shy at times. she was raised well and is very polite and well-mannered, referring to everyone (and everything!) respectively.
if we dig deeper, fuu is very confident in herself as a student, citizen and as a warrior. she knows exactly what she can and cannot do. that varies from her world to that of cephiro, as all three girls must quickly learn. she is easily flustered by compliments that she doesn't believe she deserves or that she didn't expect.
her best friend has always been her older sister. after cephiro, umi-san and hikaru-san were closer to her heart than anyone she'd ever known, as was ferio and many others. fuu doesn't strike me as much of a social butterfly, and she treasures her close friends dearly.
overall, the best word to describe hououji fuu is demure - although the word has more depth to it than the short dictionary definition. she is a very strong woman inside, but she would be the last person to realize this.