Mr. X refers to the one suspected by Akiyama Shinichi and his group in the Second Round to be the one who stole 100 Million Yen from Michiko Takada at the beginning of the round. It is first suspected to be No. 15, but is later revealed to be Fukunaga Yuji.
Mr. X in the Final Round reffered to a player who keeps disrupting voting by using a trick to acquire money. it was first thought it was Takeda Yukina, as she proclaimed it was her. However, this was a failed cover to protect herself. In reality, it was Sendou Arata.