Kurokongo Yumi

Kurokongo Yumi is a secondary character from Sky Pretty Cure ~Rainbow Star~. She visits the same school as Murasakiiro Amethyst does. Her body got controlled by Black Diamond, as seen in the first episode. And has the name Onyx (オニキス). As Onyx, her eye and hair color change and her heart becomes dark.

Friends

 * Murasakiiro Amethyst

Etymology
Kurokongo (黒金剛) - Kuro (黒) means "black", a reference to her second theme color. Kongo (金剛) means diamond. Together, her name means "black diamond" which is the season's second main villain's name.

Yumi (由美) - Yu (由) means reason and Mi (美) means beauty.

Onyx (オニキス) - Onyx is a banded variety of chalcedony. The colors of its bands range from white to almost every color. Commonly, specimens of onyx contain bands of black and/or white.

Trivia

 * Yumi has the same theme colors as Murasakiiro Amethyst has.