Kimi ga Shinu made Koi o Shitai

Death lurks behind a mysterious orphanage where children train to become magical weapons of war. Among them is Sheena, who longs to stop the fighting and end the conflict. On one gruesome night, Sheena meets a strange girl covered in blood, with a smile on her face. The mysterious girl turns out to be a secret weapon—an immortal child soldier named Mimi. When the two become roommates, there is growing tension and attraction. How will the peace-loving Sheena live with someone who welcomes death?

Source: Kodansha USA, Vol. 1

Original Manga: Ichijin Plus, PixivYurihime