Medaka

Medaka 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