300 Million Yen High School Student

Mitsuki is a high school student who works hard for the sake of his sick younger sister. However, without any parents, money, or knowledge, the only thing that comes to him is to work for an illegal business or a part time job maliciously exploiting and preying on the weak. Just when he's about to despair at the unreasonableness of the world, a beautiful woman who calls herself a detective appears. She decides to change Mitsuki's life of poverty...! (Source: Kodansha, translated)