Wu Zhu is the servant of Fan Xian's biological mother.

Upon his mother's death, which was caused by the current emperor of Qing, Wu Zhu took Fan Xian and escaped the wrath of the entire capital, to find Chen Ping Ping, head of the Overwatch Council, who suggested him to take Fan Xian to a place called Dan Zhou, as part of his plan to deceive Xiao En that Fan Xian was actually his grandson. 

Fan Xian said that Wu Zhu should find something he wants to do other than protect him, so Wu Zhu takes out the box Fan Xian's mother left behind and searches for the key to open it with Fan Xian.