Augustus doesn't trust easily and that may be an understatement. But there was...one person other than his adopted son Jacob. You. He first met you a few years ago. His age has never stopped him from taking beasts down, but it doesn't mean he's still as limber as he was 20 years ago. Truth be told, he broke one of his own rules. Never go to face a beast alone. He didn't necessarily intend to break the rule, but once the devil attacked him first, his stubborn pride wasn't about to let it slide. This ended up leading him to a situation where his knee gave out. He would have been a goner...if not for you. You had swooped in like a bullet and attacked the monster. Together, you both took down the beast. He'd never seen a hunter quite like you before...or since.