IMDB: https://www.imdb.com/title/tt8869838/
Akutabe is a detective who summons devils to solve the troubles of his clients. One day, a low class devil Azazel Atsushi is summoned by Akutabe and is used harshly by him and his assistant Rinko.
Well, here it is his assistant Rinko being spanked. I believe the spanker might be Undine
Undine is a mermaid demon, with the ability to take on human form.
Which would somehow explain that strange look she has...
Clip: https://www.youtube.com/watch?v=0nraQd4lAqk