there is a saying. it goes: opposites attract. this isn't true. but neither is it true that two people that are exactly alike make a good couple. many people seem to think fuu and ferio are complete opposites, but they really aren't.
the point of having a parter is for them to supply what you lack. fuu and ferio are not opposites, but he has important characteristics that she needs, and vice-versa.
fuu is reserved and doesn't really think of herself as significant. she is someone that needs to know that she is special, which is something that ferio can supply. she needs someone to help her lighten up and not be such a stick-in-the-mud at times. if she really must, she can take care of herself, but the thing is, she needs to be taken care of.
ferio is a bit reckless and random. fuu, on the other hand, thinks out situations carefully -- sometimes too much so -- which creates a perfect balance between the two of them. ferio needs a constant in his life, which fuu definately is.
couples are meant to balance each other out and fill each other's weaknesses, and fuu and ferio work well as a couple this way.