Ao no Binetsu

From HaruHime:

The story sets place on a certain southern island. Kakeru, Mana’s stepbrother, has been demanding her body for two years now. One day, Mana goes swimming in the ocean and meets Akihiro. Akihiro transfers into her class, and the two start going out. Although she is pulled in my Akihiro’s bright and kind personality, her frightening life that she spends with Kakeru ceases to change, and the secrecy makes her suffer. On the other side of the situation, Akihiro senses something isn’t right. Mana and Akihiro go to a summer festival, where Kakeru shows up with his childhood friend Ayano. Upon seeing Kakeru follow Mana, Ayano asks Akihiro, “Don’t you think that Kakeru likes Mana?” Akihiro pulls Kakeru away from Mana, and declares, “Mana is my girlfriend.” However, an angered Kakeru knocks Akihiro down, injures him, and before Akihiro’s eyes begins to hold Mana…