Naruto: From the Start, I Got a Hero Pool, and the Rest Is All About Grinding

One ordinary afternoon, Nawaki discovered a mysterious, ancient pool inside his mind. It was called the Hero Pool. This discovery finally helped him understand why he'd been suffering from inexplicable headaches lately. He also discovered that the harder he trained, the faster the heroes sealed inside the Hero Pool would issue him new hiring missions. And as long as he completed the corresponding mission, Nawaki, the one who had “hired” them, could randomly draw one of a hero's abilities. “Han... Han Xin?” Nawaki slowly read the hero's name, his expression hard to place. “...What a strange name!” (Not a transmigration story. Enter at your own discretion.)