U is for Undertones
Undertone: a low or subdued tone, as of utterance. An underlying quality or element, or an undercurrent.
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Monk had always known there was obviously something more to 'Kazuya Shibuya' and his assistant. Well, he was more interested in Naru's secrets than Lin's, because he believed that Lin would kill him for looking up his history.
It wasn't until he truly started digging (with the help of his detectives in arms) that it became abnormal. It was all so strange to begin, but he didn't realize how far it would all go.
In the end, after everything was uncovered, Monk had to think if it would have been better not to go wandering around in someone else's past.