It seems that Niimi was an old comrade of Kamo SERIZAWA, the head of the Rokubangumi (the sixth group) who were also from Mito, but it is unknown how they were related to each other.
同じ水戸出身で六番組小頭の芹沢鴨のかねてからの同志とされるが、芹沢との具体的な関係は不明である。