Out of all of his brothers, Karamatsu is the only one who refuses to truly acknowledge Osomatsu as his older brother - his 'onii-san'. Osomatsu is not okay with this at all, seeing as he is Karamatsu's big bro. So, because of this, Osomatsu tries to get Karamatsu to submit to recognizing him as his one and only 'onii-chan' - through various plans and shenanigans of his of course!