I'm such a big fan of this manga now < 3
Kanna from 20th Century boys
I can't decide whether she's my favorite female character ever, or just one of them. Either way, she's up there, and she more than deserved a stamp.
20th Century boys, if you don't know, is a manga written and illustrated by the fantastic Naoki Urasawa, who also created Monster.
It's a 'save the world' story brought up to a whole other level. Basically, an cult, called the 'Friend' organization, arrises. Our protagonist, Kenji, soon finds out a few things about the organization. 1. His sister is involved. 2. All of the things 'Friend' is doing ties into his childhood, it's a story they made up as kids brought to life, and 3. 'Friend' is almost certainly one of Kenji's former classmates. The events are happening quickly, the virus is spreading, draining people's blood from their bodies, and 'Friend' isn't anywhere near finished.
Now, on the subject of Kanna herself. She is a very strong character. This girl simply walks into Mordor. I don't want to give away spoilers, so read the damn manga.
20th Century boys was written and illustrated by Naoki Urasawa. It is available in North America from Viz Media.