Servant of the Goddess

The "servant of the goddess" is an unknown Sheikah character mentioned in The Legend of Zelda: Skyward Sword. She is the person who took Zelda when Ghirahim were chasing her. She will play an important role in the main plot of the game and also it's speculated to be the personal protector of Zelda.

Theories
Since it's first appearance in an offical trailer, fans started to speculate that she is supposed to be Impa, but this is not officially confirmed by Nintendo.

Trivia

 * In recent screenshots of the game, she wears a cloak that covers her mouth similar to Sheik.