
That yakuza boss. Konatsu was trolling at the end. Kikuhiko said he would never take another woman after Miyokichi. Also he viewed Konatsu as his child (which you will see later) even when she showed signs of liking him. He would have never betrayed his words and betrayed Sukeroku (Konatsu's father) by sleeping with Konatsu. Especially as an old man. He was already old when little Shin was born. People argue that when little Shin is older, he looks like Kikuhiko, but that's not a resemblance because of blood. It's a resemblance because he looks up to his adoptive grandfather.

I believe that Shin is Bon's kid. I know it's revolting to think they have done it but I believe it's not entirely impossible and with their relationship, it's totally possible. Bon is doing his best to make Konatsu his daughter but he constantly needed to make her hate him, he also has a lot of roles in Konatsu's life so I'm not surprised Konatsu getting all confused with her feelings. I imagine what happened to Konatsu was the classic "the more you hate, the more you love" thing.
I also think they had sex not because they "loved" each other. I thought it was planned but how they got into it was sudden. I believe that the anime is done exactly how it is in the manga and with all the hints leading to confirm that, I really think Shin is Bon's.
Bon had nothing. All he had was Shin/Sukeroku's, given to him because he died, esp. Rakugo and Konatsu. So to think Shin is Bon's, that would mean Bon had one thing he can call his own before he died, his son, Shin.

Him ever sleeping with Konatsu goes against his characterization up until that point. Volumes of chapters of characterization. I saw a raw of a later chapter where SPOILER:
Kikuhiko is talking to someone, I forgot who, but I think the yakuza boss and who points out Konatsu has romantic feelings for Kikuhiko and it's clear Kikuhiko knows. He seems very bothered by this fact. And the guy says he knows Kiku can't see her in that way because he views Konatsu as his child and because she's the child of the person he loved (some people might think this is Miyokichi, but I think it's Sukeroku). And Kikuhiko's expression and statements seem to confirm this. Later, he talks to Konatsu and treats her coldly and basically rejects her and leaves her crying. And then I think he says something to the dead Sukeroku which basically confirms that he views her as a daughter.
This is as a young or middle aged man. So why when he is older and should be wiser and more mature would he suddenly change what he's stood by and decide to sleep with her or impregnate her?
In the really, really unlikely chance that little Shin is his son, it'd be because it's his way of connecting with Sukeroku in a way he couldn't. And keeping a piece of Sukeroku who is now gone. Because Sukeroku's blood would be in Shin as well as his blood. But to do that for those selfish (taking advantage of Konatsu) and kind of delusional reasons is sort of yandere. But I don't think old man Kiku was delusional or yandere. Also, again, volumes of chapters of characterization.

I saw that chapter you mentioned. It's an extra chapter, isn't it? I had the same discussion at YT and the other guy agrees with me, and gave me that chapter for discussion. And yes, I agree, the person he loved is Sukeroku, like come on, my fujoshi blood's telling me by the point they met at the countryside, they'd make a beautiful gay family, if only they'd leave Miyo behind.
Anyway, I totally get your point. I also thought it was impossible given Kiku's character throughout the series (again assuming that the anime adapted the manga as faithfully as they could, since I didn't read even half of the manga). But Kiku was never honest. Given he was a storyteller, he made his own, by telling a fake story on how Miyo and Suke died, to Yota, it was to protect Konatsu but still it says what kind of a storyteller he is. He tried his best to be a bad grandpa to Young Shin, yes there were times he was soft on the kid but he tried his best not to, like when the kid pleaded he performs Jugemu instead he performed a bawdy piece, not suitable for young audiences, plus he was relatively cold towards the kid in the earlier episodes. Now, yes one may say that Kiku just isn't into kids, but I think even the most cold-hearted person would melt if they saw Shin, I think Kiku would IF only he wasn't his kid. IF Shin wasn't his, Shin would bring harmony between Kiku and Konatsu, Kiku would be a full on grandpa and dote on Shin, Konatsu would stop hating Kiku, I imagine this happening right away within the first year Shin was born.
BUT Kiku did not change, at least within the 1st year Shin was born (S02E01), he even went more cold. I think it is because the kid is the result of that heinous deed, and Kiku is reminded of it, every single time.
Now how did it even happen? See, I think it was planned. Konatsu pitched the idea, (why? I can think of several reasons) and of course Kiku would reject that idea right away. And Kiku most likely never explicitly agreed to it. So how the hell they had sex? My best guess was the Rakugo Lullaby scene in S02E01. The same thing happened a year ago that bore Shin. I would make this a longer post if I were to dissect that scene.
Kiku's relationship is very contradicting. He should take care of Konatsu, as if he is her father, well because HE IS her adoptive father BUT at the same time, he needs to maintain the role he took - Shin's killer. About 10 years later (the extra chapter), Kiku is thinking that it actually had an opposite effect on Konatsu, but he still kept on making her hate him, thus they never really reached a father-daughter relationship, 10 years after Shin's death and years after that extra chapter.
Just saw the second season...Tears aren't stopping to fall,a person's life was so cruel to him whether he obtained everything he still was empty handed were he got stolen of his precious thing but in the end he meet the one but then also he still had leave him behind while only having a promise.
Bon had so much but nothing at all, whereas Shin had whatever he wanted because he followed his heart.If I compare three of them...I ended up seeing none could achieve the true satisfaction which they wanted but only a cruel or dreadful end.
If Yotaro hadn't came then there wouldn't have any moving forward.Bon would kill rakugo and wouldn't care as he only wanted to fulfill his duty and promises but if there was no-one.Konatsu hate towards Bon would never dissolve.
Indeed there is no doubt that it is a masterpiece.It can be related to any age and time as it portrays the ugliness of life and the ending might be not as bad as the journey.
Do tell me who is the father of young shin and bon