Moreover, Double Leaf had long been suspicious of her mentor's boyfriend, the one who had enchanted Diana, Frank Hughes.
And 'Tan Mo', this person, would absolutely be considered by Double Leaf as one of the possible answers to that suspicion, no doubt about it.
Under such a highly specific and entirely correct suspicion, Motan had no doubt that even the slightest mistake on his part would lead to exposure and death.
Not only that...
Putting aside the deadly connection of Frank equating to Tan Mo, the disguise of Frank Hughes itself wasn't exactly safe. Based on his own investigation and the intelligence mentioned in Diana's letters, Motan thought Kenneth A. Achizolti, the [Mist Moon Sage], wouldn't have much fondness for him either. Although he probably wouldn't personally act to destroy such a minor figure, there's no ruling out the possibility of hiring an assassin to kill him, perhaps with a restriction not to act in front of Diana.
