We all know that Blackbeard is a master schemer, he can think 10 steps ahead. Thus, I think he has infiltrated one of the strongest pirate crews, the Beast Pirates.
12 years ago, Who’s Who made a mistake of losing Gomu Gomu no Mi, so 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 the Impel Down, he freed few Level 6 prisoners and headed to the Marineford.
At the same time, Blackbeard broke into level 6 and told prisoners to fight to death, survivors will become part of his crew. Who’s Who was one of the survivors, and became a part of his crew.
Current, Blackbeard pirates consist of Admiral Blackbeard and 10 Titanic Captains that were once prisoners of impel down, but we only know 9 of them.
Who’s Who was also the first one to notice X-Drake was a spy for World Government.
In Chapter 925, Blackbeard seems to know a lot about Luffy and rest of the “rookies” in Wano, even though information about Wano Kuni isn’t accessible through newspapers, meaning he had a spy inside Wano who gave him information.
There is a high possibility that Who’s Who is 10th Titanic Captain and a spy for Blackbeard pirates.
Theory by BuggyDClown.