Hitomi does not have a canonical last name. That Buchfink crap was just something that, if I remember correctly, was started by some Hitomi fan (an artist on one of those sites like DeviantArt, Tumblr or something) that ended up spreading somewhat.
Hitomi's name is very likely to be Buchfink. On her soccer jersey, the name Buchfink was printed on the back. I also remember Leifang's name in traditional chinese characters was shown on her underwear, 方麗鳳(Fang Li Feng).
oh wait...