Yusuke from Yu Yu Hakusho. Because he died at the very beginning D=
...I kid, I kid XD Um, but in all seriousness, I'd say Naomi Misora from Death Note. She was so interesting, imho,
and she only got, like, a chapter and then she was gone forever. I think the story could have been interesting with such
a strong, intelligent female on the side of the law (notice that L's investigation team only had men on it, and
that the only woman besides Naomi to appear on the side of the law -besides Wedy, who was, in fact, a crook L happened
to know- was Halle Lidner, and she was shown to be playing both sides, giving Mello information and the like). The rest
of the female characters are either innocents or Light's second Kiras =\ So I think it would have been awesome to
see some more of Misora before she was killed off.
I know most people say L when talking about Death Note, but having read the manga first, the way the entire
volume/chapters leading up to L's death went, Light thinks to himself at least three times "I gotta kill
him" and things started to pile up that demanded something decisive happen in that struggle between the two. I
hated to see him die, but after all the signs in the chapters leading up to it, I would have been surprised if nothing
big had happened.