Aine Yuuki, a regular student in the normal division of Star Harmony Academy, meets Mio Minato from the idol division, who invites her to join Aikatsu to help Aine achieve her goal of making friends. She also befriends Maika Chōno and Ema Hinata, who are also idols. Aine and Mio form a pair to become best friends and become the bright "Diamond Friends". Karen Kamishiro and Mirai Asuku are categorized as "Diamond Friends" and are in the Diamond Class!