Atsushi's tiger is the legendary White Tiger of the WestIt has all the traits of the mythical creature, and it is highly sought-after. It is said that it would only appear if there was a ruler with absolute virtue, which you could say of Fukuzawa. Granted, Yokohama isn't located in the west, but Atsushi does have eyes looking like the sunset (the sun sets in the west) and his first meeting with someone from the Agency was at sunset (with Osamu Dazai, who wrote a book called Setting Sun).
Sigma's real name is Sergey Nikolaevich SyromyatnikovThe real-life writer wrote with this pseudonym. The reason why this name is unknown, is because he literally doesn't remember.
Jouno's precise power is to sense liesHis Super Senses aside, he is said to be an expert interrogator. Such a power is textbook interrogator.
Fyodor can manipulate and/or halt brain processes with a touchIt explains what he did to Hawthorne (brainwashing to the point of a one-track-mind) and Goncharov (the removing of part of his emotion centre). He could kill with this by shutting down all brain processes in someone's body (blood circulation, breathing, etc.), making them unable to function anymore and killing them. As of July 3, it is also revealed that Sigma lost all of his memories, which could very well be Fyodor's doing - if he can mess with emotions and with the sense of will, why wouldn't he be able to mess with memories?
The person the Mafia actually wants to have on their side when they made the deal, is Tanizaki - not YosanoHe's young (about Atsushi's age) and easy to manipulate. The mafia has seen his power before, and he almost manages to kill Mori with it. Although Mori knew of his power back in Anne's Room, he is still surprised that the Fukuzawa he's been sparring with in chapter 31 is not real. The illusions Tanizaki makes are as such not purely visionary and auditary - they can perform powerful melee attacks as well. On top of all that, the mafia knows what sets him off - simply threaten to hurt his sister. Tanizaki has proved time to time that his power, although he says himself that it isn't suited for fighting, can be turned into a dangerous weapon. Naomi, on the other hand, has shown some quick mind as well.
Speaking about the Tanizaki's, they're not siblings; they're a couple /pretending/ to be siblingsIn the original Chijin no Ai novel, Jouji and Naomi pretend they're siblings when they are engaged. It doesn't fool anyone, though, and the character based off Naomi is very similar to her in that she's spoiled and willful. On top of that, in the earlier chapters, Naomi even says that they're siblings but not actually related by blood (a paradox, maybe?).
- That could be the case if one of their respective parents married each other, making them siblings by marriage, not by blood.