He is always there, he knows everything. He is infallible but always failing. When the boys rescue him, he is typically emaciated, dehydrated, semi-conscious, delirious; they must succor him with candy bars and water. He can take on any shape but reveals his identity within moments of doing so. He never discusses a case except for the one he's working on in a given novel, so that his legendary close-mouthedness turns to garrulousness when a "Hardy Boys" novel begins, which is of course the only time we ever get to see him. All the same, he only discusses the case in enough detail to mislead his sons and put them in mortal danger. He has systems of information and data-gathering that put the FBI to shame, yet he is always losing his case notes, his ciphers, his microfilm, or some other valuable clue, usually by leaving it in his extra pair of pants, meaning that the Boys have to drive to Canada or Florida or somewhere to retrieve it. I suppose he isn't mysterious at all; he simply embodies what many think of their own fathers: utterly powerful, contemptibly inept.