Princess Tutu

Princess Tutu is a magical girl anime created by Ikuko Itoh in 2002 for animation studio Hal Film Maker. It was adapted as a 2-volume manga illustrated by Mizuo Shinonome. Both series are licensed in North America by ADV Films.
Princess Tutu includes elements of fairy tales and famous ballets, particularly Tchaikovsky's Swan Lake.
The series follows Duck, a duck who has transformed into a young girl
who attends school and takes ballet, and her efforts to restore the
heart of her mysterious schoolmate Mytho by transforming into Princess
Tutu. The series explores the consequences of her actions and how they
affect her and those around her, particularly Duck's classmate Rue, who
is in love with Mytho, and Fakir, Mytho's childhood friend. It becomes
apparent that Duck, Rue, Mytho, and Fakir are meant to play out the
roles of characters in a story written by a writer named Drosselmeyer,
but the characters begin to resist the constraints of their assigned
fates and they fight to keep the story from becoming a tragedy.