Just going to talk a bit about how I see what's going on between them.
I'll start by noting that Choromatsu doesn't have a very good opinion of the brothers. He genuinely thinks he's better than everyone else, and based on the series, Osomatsu and Jyushimatsu irritate him, he has a neutral-negative attitude towards Karamatsu, and with Ichimatsu it's like they're not related. He probably has a claim against Todomatsu as well, but it feels like he has a more loyal attitude towards him.
I think it started from adolescence (or maybe even childhood, I just poser haven't watched Osomatsu-kun). The period where Totty is dependent on his brothers, and Choro cares for him more than the others. I think we can distinguish 2 main reasons why:
1. He considers himself the most responsible and proper brother, and since the two older ones don't have the time to coddle Todomatsu, for various reasons, the role of babysitter is taken over by the third.
I think Osomatsu is too carefree most of the time. And Karamatsu is too focused on himself and his growth as a person until graduation, he still loves his brothers but misses some things.
2. He likes to take care of Totty. By "babysitter" I don't mean that literally, but he really lets Todomatsu do too much, not realizing that it makes them both look like freaks. Speaking of reason... Choro has an obvious type for cute girls, which by the time he's 18 has probably already started to form, but that love probably extends to everything else as well. Simply put, he likes cute things. (He could be a brony, come to think of it). Todomatsu can be called cute, to a certain extent. Even if he's more of a cringe, it's obvious why he chose the strategy of making himself look like a kid. And Choro is the one it works on.
Now comes the fun part.
Brothers grow up. Totty no longer needs overprotaction (well, almost), he has made conclusions from his experience and changed for the better (for himself). Choromatsu has essentially stayed the same. Now the younger one has realized that his brother is a jerk and a blabbermouth. Todo pisses off all the brothers from time to time, but Choromatsu especially throws wood on the fire. Meanwhile, Choromatsu still sympathizes with the younger one more than the others. First, for old memories, and second, they both often act as "voices of reason". Choro dislikes his "dumb" brothers, considering himself a genius himself.
designs for a Transformers horror AU thing I was working on but shelved for the time being to work on my other main tf au project.
Thought I just shared these freaks without context. rest of the art and writing for this project prob not gonna be posted until/if I start working on it again.
Bonus little dude, he a part of a diagram explaining some lore but it doesn't make a ton of sense without knowing other lore from this AU I haven't shared.
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