If you want to know who Impa is, the short answer is that she's Princess Zelda
's The Legend of Zelda
universe. The long answer can be quite confusing, as the Zelda
storyline can contradict itself at times. What follows is an outline of Impa's past as given in the official Nintendo
chronology of the series.
According to The Legend of Zelda: Ocarina of Time there was once a tribe of warriors that had sworn their alleigance to protect the royal family of Hyrule. This tribe, called the Shekiah, carried on their business in a mysterious yet productive manner. Impa, through feats and adventures that have not been expanded on in the storyline, became the greatest of all the Shekiah and was assigned to care for the young Zelda. Impa grew up in nearby Karkariko Village and eventually her childhood home became somewhat of a tourist attraction. It was her reforms that helped the town to prosper and grow. Impa first met Link when she was ordered by Zelda to escort him from the castle grounds for his own safety.
One evening the legendary male Gerudo, Ganondorf, double-crossed the king and attempted to steal the Triforce from the Sacred Land by doing horrid things to Zelda. Impa snatched Zelda up and the two escaped on horseback. They rode right by a stunned Link and it is at this encounter that the young boy and the great evil met for the first time. Impa and Zelda escaped, but Ganondorf used Link's naivete to access the Triforce. Impa is not seen for the rest of the game.
In The Legend of Zelda: Oracle of Seasons an old woman comes forth to Link and claims to be Impa. She gives Link hints and clues as to how to fix the chaotic seasons that the land of Holodrum finds itself in. Zelda had assigned her to take on the quest, but she was injured in the attack by the evil Onox. In The Legend of Zelda: Oracle of Ages an elderly woman claiming to be Impa was actually the evil Veran in disguise.
By the time The Legend of Zelda rolls around Impa tells Link of the evil Ganon and the kidnapping of Zelda that spurns Link to go on another grand adventure. In Zelda II: The Adventure of Link she tells Link of the sleep spell the princess has been put under and urges him to return the six crystals to the six palaces across Hyrule. She is not seen in either game, but she is mentioned in both instruction manuals in the story portions. Overall her main role in the Zelda games is to drive the storyline forward.