Diamond, whose true name remains unknown, is Amu's fourth Guardian Character, distinguished by her musical talent. She transformed into an X Egg when Amu felt overwhelmed by the changes happening around her. Later, Utau found her egg and performed a character transformation with her as Dark Jewel. Since Amu was unaware of Dia's real name, she simply called the Guardian Character "Dia," a shortened version of "Diamond."
When Amu convinces Dia that she can change, the "X-mark" disappears, and they undergo a Character Transformation to become "Amulet Diamond." Initially, Dia didn't like Amu because she believed Amu lacked the sparkling heart that Utau possessed. However, Dia later discovers that Amu does have such a heart and returns to her. Afterward, Dia goes back into her egg to rest until Amu "shines again." Although she rarely appears, Dia does come out in some episodes from time to time.
