Medaka (メダカ) is a young girl, from the OVA, who wears an oversized suit of armour and wields a naginata to try and combat the Ganzack Pirates who have taken over her island. Her father was one of those taken prisoner by Ganzack and forced to build his base and pirate ship. During the final battle between Luffy and Ganzack, she is struck by Ganzack. Luffy, believing her dead, manages to defeat Ganzack in a righteous fury. She survives and gives Luffy a lifesaver as a parting gift for saving her home.  Source: http://onepiece.wikia.com/wiki/Medaka

While Luffy and his crew of Zoro and Nami are starving on their small boat, they are attacked by a large monster. Nami is taken away, while Luffy and Zoro wash up on shore. There they meet a young girl, Medaka, and learn of the sad history of the island. The evil Pirate Ganzack has taken away all the men in the village and enslaved them, including Medaka's father. Now Luffy, Zoro, and Medaka must infiltrate Ganzack's base in order to rescue the villagers and Nami. <br><br>
(Source: Anime News Network)

While Luffy and his crew of Zoro and Nami are starving on their small boat, they are attacked by a large monster. Nami is taken away, while Luffy and Zoro wash up on shore. There they meet a young girl, Medaka, and learn of the sad history of the island. The evil Pirate Ganzack has taken away all the men in the village and enslaved them, including Medaka's father. Now Luffy, Zoro, and Medaka must infiltrate Ganzack's base in order to rescue the villagers and Nami.
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(Source: Anime News Network)