Ahiru 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 pendant 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, 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 and happiness.

Spoiler

In the end of the manga, she does not die or turn into a duck. She forms a friendship with Rue, Fakir, and Mytho - who she still loves.