D_CIDE TRAUMEREI THE ANIMATION

As a child, Ryuhei looked up to his older brother, but watched him perish in a mysterious incident right before his eyes. Now a high schooler, Ryuhei shows no outward sings of the trauma of that nightmarish incident, and spends most of his leisure time kickboxing. One day, while kickboxing, he encounters a creature that calls itself Tris, and is bitten by it, resulting in him having a strange dream.

Synopsis from Anime News Network