Personally, I think that her hair looks better down. Preferably, down while holding her dog, Flak Hayate.
That's a picture I saw.
Don't you mean, Black
Hayate, Twilight Avenger?
I like them both, but I think she looks really beautiful either way. Her hair pulled back gives her that strict, somewhat cold, firmness of the lady soldier. When she wears her hair down, she gives that sweet, tender, feminine touch about her. Either way, Riza is gorgeous.