Artist.
Japanese name: 栗原 ケンシロウ (Kurihara Kenshirou)
Circle name: 拳王親衛隊 (Ken'ou Shin'eitai)
He is also a member of Bunjutsu Fortissimo, a doujin circle led by fanfic writer Miura Sairasu. Sairasu often uploads the illustrations of other circle members to his sites/accounts with their permissions. The real artist credits are on his Pixiv commentaries.
Miura's accounts: