sunny boy books

50 years ago a gate appeared that spawned hordes of monsters that resulted in humanity losing over 96% of its population in a disaster named the first awakening. Now remains about 150 million people who have all taken shelter within strongholds in Japan, USA and Korea. With all the other nations destroyed and overrun by monsters these three remaining countries gave survivors a place to live. The strongholds were made of foreign metal that came with these monsters that was strong enough to withstand a nuclear bomb so they were supposedly safe, however 6 years ago similar gates appeared within the strongholds causing mass panic. However, as it happens many people awakened abilities that gave them the edge over those monsters, those people are called hunters. The hunters all had a special system that allowed them to level up their abilities. Everyone started as E ranks and depending on their willpower, perseverance and experience their leveling up ability was stronger.