Kuina

Kuina (くいな) was the childhood friend of Roronoa Zoro. Throughout her life, Kuina pushed herself harder then usual in kendo in order to be accepted in the male-dominated discipline. Even her father thought that she could never be the best because she was a girl. While training in the same dojo, Kuina was the only person Zoro was never able to beat in a sword fight. After overhearing what her father said about girls not being good at kendo one night, she rushed to find Zoro and challenged him to a duel, with real swords. Though she defeated Zoro again (she was the best in the dojo, he was second-best), she still felt like everyone was looking down on her just because she was a girl. So she and Zoro make a promise that they would continue to build their skills until they were the best swordsmen in the world, after which they would fight each other again for the title of strongest. Original owner of Zoro's white sword, Watto. Kuina dies the very next day by falling down some stairs, breaking her neck. Zoro takes her sword Watto for his own and vows to become the best swordsmen in the world in her place