Cherry Bomb: Chou-ojou-sama Sayaka-chan Nanpa Daisakusen

Young girl Sayaka has transfers to main character's class. She is the only daughter of a powerful corporation and a returnee from France. Protagonist tries to conquer her heart, but is stopped all the time by the girl bodyguards. At such time he finds a mysterious gem. When he touches it, a voice is heard from nowhere...

[From vndbreview]