A Loser? No, I'm Irresistible!

Several hundred years ago monsters came down on the world and nearly wiped out the human race. The survivors shut themselves inside fortress cities with high walls, and inside those walls a person's rank depends on whether they have a special ability. Jiang You has none. People treat him as dead weight and say so to his face. Then a system wakes up inside him. It has a shop that sells battle armor. What the system feeds on is other people's goodwill, and the more people come to like him, the stronger he gets. The armor in the shop comes from somewhere outside his own world. He takes a suit of nano armor from it and starts fighting the monsters beyond the wall. He also has enemies inside it, among the people with abilities who spent years treating him as trash.