Tetra is a girl pirate who, in The Wind Waker, helps Link start his journey. Her mother died a few years before the events of The Wind Waker, and she was left to lead the group of pirates. She is the one that Ganondorf intended the Helmaroc King to kidnap, instead of Aryll. She periodically helps Link out during his quest, and later her true identity is revealed in Hyrule Castle: she is Princess Zelda, the last heiress in the Hylian Royal Family bloodline. Before this, Tetra did not know she was Zelda, yet knew of Hyrule, the legend of the Hero of Time, and the Master Sword, and wore a large piece of the Triforce of Wisdom around her neck as a necklace that was from her mom. (Source: Wikipedia)
