Gunjou - Ai ga Shizunda Umi no IroGunjou (KANESADA Yukio); 群青 愛が沈んだ海の色

From Midnight Scans: Even though they played together innocently since they were young, somewhere along the way, the seeds of "love" started to grow. Growing up on an outlying island from Okinawa, Kazuya, Ryoko and Daisuke were brought up like a family. They wanted to be together forever, but the three of them have different desires. One wants to stay, one wants to go, and one of them wants to go, but can't. When Daisuke is lured away from the island, Ryoko decides to stay on the island with Kazuya. However, the three of them will experience a cruel fate...!