This page refers to the adult, Osomatsu-San version of Karamatsu. Click here for the Osomatsu-Kun version, where the Matsuno sextuplets are 10-year-old fifth graders instead of NEETs.
Karamatsu Matsuno is the second son of the Matsuno sextuplets.
Similar to Canon!Matsu, he's the cool guy wannabe who speaks poetically, yet everyone tends to ignore him. From there, however, he diverts.
He's a sweetheart who just wants to be treated equally to his brothers. He tries his very best to be the best man he can, even if others think it's painful, cringe-worthy, shallow, or even straight-up stupid. His script is his life story, as he practically lives it... Those in The Place don't seem bothered by it, as nobody's raised a fist.
To learn more about the Osomatsu-San timeline, just watch the anime. There are around 25 canonical episodes, along with a few bonus ones, such as Episode 3.5, which you can find in the above link, along with Episode 26 (which has been rewritten under Osomatsu's request).