Thank you for saying this.... this has been irking me since he appeared. That is such a strong super power and should make investigating so much easier, especially in this case... If his power isn't limited, shouldnt he be talking to every witness, potential culprit and so on? He would be the perfect person to work for the police or in a courtroom. I hope the author has a good explanation for this later, because it really doesn't make sense at this point - they needed someone to get between the two mains, and threaten their relationship with this "if you help out, Jupiter will become your personal guide" thing - okay, but couldn't it have been another super power, if they're not going to actually use him to solve the case?!
Maaaybe they could get away with it, if they write it so the people involved are so manipulated that they don't even know they're lying and thus making his ability useless in this case. But for now it just feels like a major plothole...
