300 Million Yen High School Student summary:

Mitsuki is a high school student who works hard to support his sick sister. However, without parents, money, or any form of specialty, he falls into working for an illegal business, ends up getting devoured by the strong as someone who is weak, and is ultimately mugged.Deep in despair from the cruelty of this world, a beautiful women who calls herself a detective appears. With her help, it's possible Mitsuki can turn his life around...!