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.

~!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.!~

In a fairy tale come to life, the clumsy, sweet, and gentle Ahiru (Japanese for 'duck') seems like an unlikely protagonist. In reality, Ahiru is just as magical as the talking cats and crocodiles that inhabit her town - for Ahiru really is a duck! Transformed by the mysterious Drosselmeyer into a human girl, Ahiru soon learns the reason for her existence. Using her magical egg-shaped pendant, Ahiru can transform into Princess Tutu - a beautiful and talented ballet dancer whose dances relieve people of the turmoil in their hearts. With her newfound ability, Ahiru accepts the challenge of collecting the lost shards of her prince's heart, for long ago he had shattered it in order to seal an evil raven away for all eternity. 
Princess Tutu is a tale of heroes and their struggle against fate. Their beliefs, their feelings, and ultimately their actions will determine whether this fairy tale can reach its "happily ever after." 
 
Princess Tutu aired in two parts. The first part included 13 25-minute-long episodes, while the second part consisted of 24 12-minute-long episodes with a 25-minute-long final episode for a total of 38 episodes.

13-year-old ballet student Ahiru is clumsy, good-hearted and sweet... and has a big secret. The mysterious Drosselmayer morphed a young duck into a girl to give her a mission: help a Prince to get the parts of his heart back. With that in mind, she morphs into Princess Tutu, whose magical dances ease the pain and purifies the bad feelings. 
 
(Source: Anime News Network)

Ahiru Arima