Doujinshi circle. Crimson is also the name of the main artist herself. The illustrator of Aoi Sekai no Chuushin de.

Her works are well-known for distinctive meme-ish phrases.
For example: "くやしい...! でも...感じちゃう...!" or "ピクンピクン".

(original)

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いまだに間違えたままの方が多いようですが 現在のサークル「クリムゾン」の作家は「クリムゾン」です。 個人サークルなので サークル名と作家名を同じにしています。 初期の頃のカーマインという方は今はいないです。

(translated)
There are still many misunderstandings about my name. The current artist of "Circle Crimson" is "Crimson".
Because it's a solo circle, its name is the same as the artist.
Carmine was an early member and is no longer on this circle.

From Crimson's tweet (10:03 pm - 16 Aug 2014 [JST])

Pen-name: クリムゾン (Crimson)
Circle name: クリムゾン (Crimson) or クリムゾンコミックス (Crimson Comics)
Old circle partner: カーマイン (Carmine)