Dia is Hinamori Amu's fourth Guardian Character, whom represents Amu's desire to shine. She was originally an X-Chara, who had chosen Utau Hoshina as her new owner. She stayed with Utau for some time and helped her make Black Diamond CDs that extract X-Eggs. Her symbol is a diamond. Her real name is unknown, since everyone barely knows her. She can transform into Amulet Diamond with Amu.
Dia is barely shown in the anime since she sleeps in her egg, and comes out on occasion, but hatches for good around episode 100,and she tranforms into Amulet Diamond, then Amulet Fortune with Ran,Miki, and Su.
With Amu as Amulet Diamond, she can perform Starlight Navigation, Which can navigate something(episode 43), and in the manga, Twinkle Shield, which is a protective shield. With Amu as Amulet Fortune, she can perform Open Heart, Full Bloom! True Love, which purifies X Eggs.