Impa is a caretaker to Princess Zelda, and also the general of the Hyrulean Forces. There are many incarnations of the character named Impa throughout the Zelda series, all being of the ninja-like Sheikah Tribe, just as there are multiple incarnations of Link and Zelda. As with Link and Zelda, all incarnations of Impa share certain personality traits.
In the Legacy of Chris Thorndyke, her role in the sixth saga is quite similar in Hyrule Warriors, but bigger since that she basically had the idea of splitting up in different dimensions, since the Links are around, but still keeping on the lookout for Princess Zelda when she was maybe held captive somewhere or just disappeared without a trace from Ganondorf's army.
She is voiced by Janice Kawaye in the English dub and she is reprised by Aki Nagao in the Japanese dub.