Ahiru is a duck, who is also a girl, who is also princess Tutu; she is charged with the task of returning Prince Mytho his heart, piece by piece.
Ahiru (which translates to duck in Japanese) is in all reality, a duck herself; however, in the beginning of the series she was turned into a human girl by Drosselmeyer. He also bestowed upon her a pendent which causes Ahiru to transform into her other-self, Princess Tutu. This was all caused by her desire to see Mytho, the Prince of the story, to smile. It turns out her role as Princess Tutu is to gather Mytho's missing heart so he can gain back the many feelings of sadness & happiness.
Ahiru was created by Ikuko Itoh for the anime series Princess Tutu in 2002. Her manga incarnation was created by Mizuo Shinonome. In the anime she was played by Nanae Kato in the Japanese Track and Luci Christian in the English track.
In the first season of Princess tutu, Ahiru was searching for the Pieces of the Heart that belonged to Prince Mytho. Each episode or so, she would encounter someone holding a piece, and through dance she would allow them to express their feelings and release themselves from the hold of each piece.