The Raven is the mysterious antagonist of Drosselmeyer's story The Prince and the Raven and Mytho's dark persona because Mytho shattered his heart in order to seal up the Raven, who then requires the sacrifice of young, beautiful hearts in order to eat them and regain his form.

He stole Rue from her parents as a child, raising her as his daughter, Princess Kraehe. He orders Kraehe to corrupt Mytho with raven's blood and is cruel and abusive to her. Eventually, when Rue's love for the Prince enables him to break free from the tainted heart shard, Mytho rescues Rue and together they defeat the Raven with a final blow by using the Prince's Sword to perform a Limit Break, Final Wing Nova.
The Raven does not appear in the manga, but is briefly mentioned by Edel.

(Source: Princess Tutu Fandom)