World Ender: Sekai ni Kirawareta Kimi to no N-kaime

How many sacrifices can you make to save a girl? A world 30 years after humanity was trampled by suddenly appearing "Angels." Eugene, who works as part of a self-defense force to be happy with everyone even though his family was killed by Angels, meets a mysterious girl named Vishuki... An extreme Fantasy Battle where you are not allowed to die until you obtain happiness, begins!

(Source: Hakusensha, translated)