A 14-year-old junior high student idol. Her Character Transformation with Iru is called "Lunatic Charm". During Character Transformation and Character Change Utau grows devil wings. The other Guardian Character, Eru, was with Amu in the place of Dia. The gentle Eru seems to be treated badly. Eru performed a Character Transformation with Amu, becoming "Amulet Angel" surprising Utau. Eventually she did a Character Transformation with Eru becoming "Seraphic Charm". She is actually Ikuto's sister, Utau Tsukiyomi. But she loves him and is jealous of Amu, and as a result kisses him in episode 29. She has a brother complex and chases any woman near Ikuto. Whenever she enters Ikuto's sight, she undos her Character Transformation and by all means tries to please her brother. She becomes mushy with her brother whenever she sees him. Through the power of the Humpty Lock, she did a Character Transformation with Amu's Dia Guardian Character, thus becoming "Dark Jewel", but she cannot do this for a long time, because when doing a Chara Transformation with another person's Character, you use up more energy then usual. Also, after Amu gets Dia back, she and Amu have a better relationship after.
She is the vocalist of the indie band "Black Diamonds" which is produced by her manager, Yukari Sanjou. She also has feelings for her brother and even forcefully kissed him twice - episode 29. Soon after her battle with Amu she and Sanjo (her manager) started their business again without Easter's help to become the idol that doesn't fight for Ikuto but sings because she loves it.
In Shugo Chara! Encore! Utau falls for Kuukai, and the two end up in a romantic relationship. They kiss twice in the manga.