Shuutarou Sarashina is an erotic novelist; while mentioned several times in the original series, he never appears in it. Mostly, his books are only referenced by Kaoru, in an attempt to better act as a dominant or in order to learn certain acts of S&M.
He makes his official appearance in the Spin-off, Black Label, where he meets Kaoru by accident and invites him to help on his latest book. He and Mitsuko are lovers; Having met when Mitsuko was a college student. The bonded through S&M, but the years have taken a toll on them, and twisted their feeling for each other from love, into a darker obscure feeling of possession. However, with Nana and Kaoru's help, they both eventually come to terms with their feelings and truly admit their love for each other.