The mysterious man was Haidong Blackfield-Kuroba Touichi (now the father of the child) at that time. Reason: At the end of 473, in the conversation between Kuroba Touichi and You Xizi, 1. Kuroba Touichi said, "Zuo Zuo may receive a letter from a friend of mine soon" (this letter is the one given to Xinyi in the library. 2. Kuroba Touichi (the old guy) thinks that Xinyi will not be solved.