Seiichirou Aoki is, as far as I recall, an editor for Asuka O.o interesting product placement there, Clamp... Aoki-san fights using the power of wind. It is very hard to write that and keep a straight face... *slaps self*
I don't remember very much about him in the anime. He seemed to spend his time talking to Kasumi Karen and Nekoi Yuzuriha - I seem to recall thinking that he and Karen were in some form of relationship. He fought Nataku, and both met their end by plummeting from a building. End of Aoki. Although he did seem quite likeable.
So far, in 6 books, he isn't very prominent in the manga. He appears in book 3, 'Sonata', then seems to vanish again. However, he proves himself to be a caring man, although he looks totally unassuming and almost innocent. When Arashi creates a kekkai to fight the Curse Zombies in, Aoki is drawn in also. It's only when he's forced to fight that he shows his true power. When the fight is finished, he returns to being a harrassed editor, dashing off as his pager goes off, only to dash back again and hand Arashi his umbrella, as "Girls shouldn't stand soaking in the rain". He seems rather sweet ^_^
X Japan song: