Diamond (true name unknown) is Amu's fourth Guardian Character, with a musical flair. She became an X Egg when Amu disliked all the changes that were happening,and then hatched as an X Character after Utau finds her egg and character tranforms with her as Dark Jewel. Since Amu did not know Dia's real name, Amu just calls the Guardian Character "Dia", a shorter form of "Diamond."

When Amu convinces Dia that she can change, the "X-mark" is lifted and a Character Transformation sparks as the two become "Amulet Diamond." Dia didn't like Amu because she didn't have a sparkling heart like Utau but finds out later that Amu does so she goes back to Amu. Afterward, Dia goes back into her egg to slumber until Amu "shines again". She rarely but does come out in some episodes once in a while.