In Osomatsu-kun, Iyami is always seen wearing his red bow tie. In Osomatsu-san, the tie is inexplicably absent. Who suddenly wears a red bow tie bow that didn't have one before? Iyami's best friend, Chibita.
This would be a fair explanation as to why he smiles all the time and the things he says are sometimes quite indecipherable - Robomatsu has no idea how to behave like a human. In the original anime, Robomatsu is a mute (well, almost) so it's possible he is just getting acquainted with how to speak. Robomatsu is also known to have no expression besides his toothy smile, but in one series of frames, his mouth hangs open just like Jyushimatsu. In addition, an official piece of promo art displays Jyushimatsu with an eerily familiar toothy smile. Make of that what you will.
This does pose the question of where the real Jyushimatsu went. It's possible Robomatsu sprung back from the dead, even though he combusted in the episode he was introduced in. Maybe in tenth grade (which is supposedly when Jyushimatsu "changed"), he found the sextuplets and remembered his programming, getting rid of Jyushimatsu once and for all. He must've realized that the other five matsus would get suspicious as to why one of their brothers inexplicably disappeared, so he decided to take his place.
- Episode 24 does feature a Drama Bomb — but this theory is ultimately jossed as the final episode is very silly. The spinoffs have not undergone any change in mood, either.
This is not my own theory, but just felt that it would be nice to share, as it's gained surprising traction among Japanese viewers (so much that it extended into Pixiv) in the wake of episode 9 segment C "Jyushimatsu Falls in Love".
The basic premise is this: The entire setting of the show is a sort of Purgatory, a waypoint between the land of the living and the dead. Indeed, the entire cast passed away during the early 60s (the timeframe of the first series), but somehow or another forgot about this. While this means that they haven't been able to pass on, the time they've spent there allows them to unconsciously affect their surroundings (explaining away the comedic malleability of the place).
So where does Jyushimatsu's new girlfriend fit in all of this? Well, one of her suicide attempts succeeded in part; she's still not dead, but she's close enough that her spirit is now wandering the same place as the rest of the cast. However because she's not really dead (yet) she has enough leftover memories that all that guilt and depression that caused her suicide attempt followed her.
This all leads to the events shown in the third segment of episode 9. However, in the time Jyushimatsu spends with her, on some level he realizes that she doesn't belong there. So he lets her go.
And the moment she boards the train, she's well on the way to waking up again in the real world, but this time around, the lingering feelings she takes with her care of Jyushimatsu eases her spirits, even just a bit.
Something went wrong, though. Maybe the brothers were officially given the wrong names on all official documents and the parents decided to roll with it (stressful enough giving birth to and parenting six carbon copies of one kid already, right?). It's also possible that they were registered properly, but the kids grew up always responding to the wrong namenote .
The end result is the same; at least three of the brothers grew up with the wrong identity (Ichi and Oso, as mentioned, plus Todo who's known as the youngest), but nobody wants to set the records straight by now because it might lead to huge, unneeded identity crises.
- In a light novel in the -kun era called ‘Fujio Akatsuka Theater’, Chibita describes the sextuplets in an introduction where he states that Osomatsu is the leader of the sextuplets, but not necessarily the oldest. By calling one Matsu you call all of them, they’re equal, but then again, he wasn’t too sure which one was which or who was the oldest or youngest.
- It's doubtful Karamatsu was born with his eyebrows. In Osomatsu-kun, the sextuplets all have the same eyebrows, and even in the scenes mocking that style in Osomatsu-san, it's the same, so it isn't anything with continuity. It's actually more likely he uses eyebrow pencils.
- Actually, when the sextuplets were shown as babies in episode 17
◊, one of those babies (who is presumably Karamatsu) is shown with slightly thicker eyebrows than the other sextuplets. Mind you, there are points in the anime where his eyebrows are around the same thickness as they are when he was a baby and there are times where they're insanely thick◊ so he probably still uses eyebrow pencils from time to time
- Actually, when the sextuplets were shown as babies in episode 17
The events of "Letters" weren't simply hand waved, it actually happened but Osomatsu doesn't want to accept that it did. The reason why the last episode returned to the show's usual humor after such a dramatic turn is because it's actually a scenario created by Osomatsu so he doesn't have to accept that things are changing.
Choromatsu's letter to Osomatsu at the end of "Letters" details the reason behind his choice and his relationship with his family. After reading it, Osomatsu slowly came to realize that he can't live his life in constant denial of his adulthood. Like his brothers, to move on they may have to stop being themselves and, sadly, start being away from with each other. Of course this being Osomatsu, part of him knows it's true but he can't bring himself to accept it.
So the events of the episode were scenes created by Osomatsu's mind to help him escape from the reality of it all. With the invitational and the crazy baseball match, Osomatsu is now back to a simpler time when all the brothers' and their companions had to worry about was being silly, relentless assholes. All while in real life he's continuing to deny his own issues and loneliness as his brothers continue to grow up.
This theory could also explain the existence of characters who appear to be played by one of the regulars but are actually their own character, such as Jyushimatsu's interviewers, who are in the same canon universe as the brothers (as shown by a flashback in "Godmatsu").
- Which raises the question of why Dekapan did not care for his orphaned grandson at all when Chibita was growing up... Check Osomatsu-kun: he lived in a pipe and occasionally drifted around to different households, only to not stay for very long.
- Word of God proved this theory false, Joshimatsus are an AU.
- Which makes Season 2 Episode 8 very strange because members of the two groups actually appear alongside each other, although none appear together with their direct counterparts.
- That alternative universe see the girly-matsus as the central characters (Season 1 Episodes 13, 15, 18, 19 / Season 2 Episodes 5, 7, 8, 10)
- The Matsunos, NEET or not, are generally more inclined to work (Season 2 Episodes 8, 10, 15)
- Iyami is employed, and the country girl lives a normal life there as well (Season 2 Episode 8)
- Totoko is a newbie job seeker who likes to fool around, but is generally a nicer person than she is in the "original universe" (Season 2 Episodes 10, 15)
The Matsunos are possibly less related or have already moved out, which explains why Jyushi is not with any other brother in Season 2 Episode 8. Maybe Oso, Choro and Ichi will appear in that world as well but as different people.
