Shōnen Tantei-dan (The Boy Detectives Club) describes both the 1937 novel by renowned Japanese mystery novelist Edogawa Rampo, and the title of a series of Edogawa novels, of which the aforementioned titular novel is the second in the series (the first was The Fiend With Twenty Faces).
The Fiend With Twenty Faces is behind events that provoke Akechi, but does not personally kill people.
Gackt as the Fiend With Twenty Faces, a criminal and master of disguise whose age, gender, and nationality are unknown.
Their meeting is connected to the fate tying together Kogorō Akechi, and the era’s greatest villain, the Fiend With Twenty Faces.