akumatsu
A character that originally appeared in Osomatsu-san.
He is the personification of the Matsuno brothers' latent evil. Because of the large quantity of said evil, his physical form is a large amorphous demon-like creature. He is the antithesis of Kamimatsu.
He is usually depicted in fanart in a more humanoid form, resembling a Matsuno with red eyes/sclera and sharp teeth.