Aka no Sekai

Regulus is a cool, introverted young man forced to participate in a summer camp by his mother. Regulus is more nervous than elated--outdoor activities force students to interact more.

Despite his efforts to stay on the fringe, Regulus is caught up in trouble when a classmate paints a room an eerie, deep crimson color, sparking a huge disturbance.

Contains 4 loosely links oneshots.

(Source: the manga habit)