Blackbeard has shown himself to be an expert planner and a master at orchestrating manipulative schemes. He thinks 10 steps ahead. My theory is that he has infiltrated one of the strongest pirate crews, the Beasts Pirates.
12 Years ago, Who’s Who made the mistake of losing Gomu Gomu no Mi, thus he was imprisoned in Impel Down.
During his encounter with Jinbe, Who’s Who tells him that he has already seen him once ,when Jinbe was a Shichibukai. That can only be possible 2 years ago, when Jinbe was imprisoned, since at the time, Who’s Who was already in prison for 10 years.
2 years ago, Luffy broke into Impel Down in order to save his brother, Portgas D. Ace, from execution. Luffy did eventually break out of the prison and freed hundreds of criminals with him. At the same time Blackbeard broke into Impel Down and recruited notorious prisoners for his crew from Level 6.
Who’s Who was among the surviving prisoners and became a part of Blackbeard’s crew.
Current Blackbeard Pirates crew consists of Admiral Marshall D. Teach and 10 Titanic Captains that were once prisoners of Impel Down, but we only know 9 of them.
There is a high possibility that Who’s Who is 10th Titanic Captain and a spy for Blackbeard Pirates. It makes sense since he was already in CP9.
Who’s Who was also the first one to notice X-Drake was a spy for the World Government.. takes one to know one.
Let’s not forget that the theme of Wano is betrayal and spying. Furthermore, in Chapter 925, Blackbeard seemed to know a lot about Luffy and rest of the Supernovas in Wano ,even though information about Wano Kuni isn’t accessible through newspapers, meaning he had a spy inside Wano who gave him information.
*Theory by RoronoaLuffyZoro