Arima, Ahiru

有馬あひる

Ahiru (which translates to duck in Japanese) is, in all reality, a duck herself; however, at the beginning of the series, Drosselmeyer turned her into a human girl. He also gave her a pendant that causes Ahiru to transform into her other self, Princess Tutu. This was all caused by her desire to see Myuuto, the Prince of the story, smile. It turns out her role as Princess Tutu is to gather Myuuto's missing heart so he can regain the many feelings of sadness and happiness.
It is apparent from the start that Ahiru, as a human, duck, and princess, is feverishly in love with the Prince. Regardless, if she confesses her feelings to him, she will vanish (or so the story goes). As such, she is stuck in this awful predicament, but she always willingly goes to fetch Myuuto's heart to aid her beloved, lifeless Prince.
In the manga:
Ahiru Arima is a human girl who was given a pendant to transform into Princess Tutu. As told by Edel, she gathers shards for the Prince to help restore his heart.

Spoiler