From Tokyopop:
Protagonists Ren and Rai fell prey to the Diabolo in an attempt to save the soul of Ren's cousin, 7-year-old Mio...although they failed in their attempt, they were 'rewarded' with powers they would each gladly trade for the life of the young girl.

Just who--or what--is the Diabolo? What enticements does he offer the kids at school? And, if they accept his 'bargains,' what foul fate awaits them on their 18th birthdays?

Searching...
online