When chapter 29 came out, I wrote in my post here:
“The person writing Hirata letters into jail was Hirata’s sister and has nothing to do with the rest of the story (so far).”
After giving it some more thought, I actually take that back. I think there is a solid chance after all that Tomo was the one who wrote Hirata the letters.
First of all, Chestnut doesn’t know for sure if the sender of the letters was really Hirata’s little sister. Instead he said Hirata was a gloomy loner who didn’t get close to any of the other prisoners, so I doubt Hirata would have told Chestnut anything personal. And if we take a closer look at how Yoneda-sensei set up the events in chapter 29, it is probably not a coincidence that Tomoko shows up right on the next page after Hirata’s “little sister” was mentioned. Yoneda-sensei is an excellent storyteller, and if we go by the conventions of storytelling Tomoko’s appearance there should serve as a solid clue. It also doesn’t make much sense to me for Sensei to introduce a random character at this point who is heavily linked to the villain of the story but won’t play a role in the future chapters (well, of course we have no idea what she is planning). Whereas at this point it’s pretty safe to assume, I guess, that Tomoko will play a major role in the upcoming events. The question is how.
So why did Sensei insert the episode with Hirata’s “little sister” then? We didn’t really need yet another proof that Hirata is rotten to the core and a cold, calculating bastard. And at least for now I can’t really see any reason why Chestnut would need to know that Doumeki was in prison.
For the above reasons I will now assume that the “little sister” who wrote Hirata letters while he was in prison is in fact Tomoko, the little girl who lost her uncle, her only family, because of him and should hate him more than anybody else in the story. At least if she knew about Hirata’s betrayal. But so far nothing suggests that she does. She was very young at the time, Kurobane definitely didn’t tell her what he was about to do, and she seems to have grown up into an independant young woman who doesn’t let her past dictate her life anymore. So if she isn’t out for revenge and plotting to kill Hirata together with her assassin boyfriend ;) what other role could she play in the story?
I think she could be Hirata’s weak spot, his conscience. Why? Because it’s not like Yoneda-sensei to create one-dimensional characters, and so far Hirata has been a pretty straightforward asshole throughout the series. Sensei made us understand his reasoning but never showed us a single good side of him until now. Which has been bugging me to be honest, especially since she said somewhere that she couldn’t create villains who were just bad and nothing else. So what if Tomoko is the one person in Hirata’s life who he regrets to have hurt? He spent time with her alone when she was a young and innocent little girl who adored her uncle. He was laughing with them at the table when she said to Kurobane that she was just as scared of Hirata’s face as she was of the thunderstorm he was supposed to protect her from. It wasn’t Tomoko Hirata wanted to get rid of. And she must have (had?) fond memories of the times they spent chatting and laughing together with Kurobane.
So wouldn’t Tomoko send Hirata letters if he was one of the few adults in her life who she trusted and who reminded her of her uncle? Especially if, as far as she knows, he was the one who killed the guy who killed Kurobane, and then went to prison because of it? Wouldn’t she send Hirata letters thanking him for avenging her uncle, and trying to cheer him up? And with each new letter Hirata, who destroyed her life, would get reminded of what he did to a little innocent girl. I think (hope) even someone like him would feel at least a tiny bit of remorse, and it would distract him from pursuing his aim to replace Kurobane as Misumi’s right-hand man. The aim he sacrificed so much for and that led him to killing Kurobane in the first place. So what does Hirata do? He throws the letters away without reading them and focusses on what he is going to do after he gets released from prison. That is his way to stay sane in a place that even Doumeki couldn’t endure without the support of letters.
So my best guess at the moment is that Tomoko will be responsible for Hirata’s downfall, not because she knows what he did, but because she reminds him of what he did to her. After all it wouldn’t be fair if everyone in this Yakuza war had a weak spot except for the main villain.