Atsushi's tiger is the legendary White Tiger of the West
It 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 Syromyatnikov
The 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 lies
His Super-Senses aside, he is said to be an expert interrogator. Such a power is textbook interrogator.- Jossed.
Fyodor can manipulate and/or halt brain processes with a touch
It 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 Yosano
He'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 siblings
In 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.
Sigma is an actual person, not a creation of the Book
The one who informs us of Sigma having been written into reality is Fyodor - the least trustworthy person in the canon. As stated in a WMG above, it is possible that he wiped Sigma's memories and made it look like Sigma just popped into reality three years ago. The Book is made of the same substance as a Death Note
Both have the power to distort reality in a certain way, even though the Death Note is much more limited and ultimately ends in a death. The Book is used for dictating how people live (and not the other way around), and even to modify reality itself, so it's like some sort of Life Note on steriods. Ranpo is fully capable of using the train
What with his intellect and all, Ranpo surely is quite capable of using public transport, and do all the other myriad menial tasks he sees beneath him, if he set his mind to it. Case in point, he seemed to do perfectly well on his own during the whole Decay of the Angel. The real reason why he needs a chaperon while on a mission is because of all the dangerous criminals he is constantly facing and unmasking, all of whom probably want to gut him at one point or another. Hence, he needs someone else to watch his back; Atsushi's role at the riverside case was basically that of a bodyguard. Aya Koda has an Ability
First off, she is named after a writer. She is implied to be a Chekhov's Gunman and to be the one that's the hardest for Fukuchi to deal with. Fyodor's ability lets him impose "punishments" on people proportionate to their "crime"
It might seem a bit on the nose, but it's possible that Fyodor's ability allows him to do anything to someone he touches, so long as what he causes can be seen as fitting punishment for the person's crime. So, if the person he touches has ever killed someone, he can freely kill them, which would explain how he killed that policeman who tried cuffing him assuming that person has ever killed someone in the line of duty. Alternatively, if the officer never actually killed anyone but merely injured them, which is more likely, Fyodor can impose a injury on them, even a fatal one.