Monster Musume Vol. 5 (manga)

"Get married and I kill you." This was the letter sent to Kimihito, an ordinary guy living with a houseful of extraordinary beauties, by a mysterious writer known only as "D." Now it's up to Interspecies Coordinator Smith, Kimihito, and his ever-increasing menagerie of monster girls to find out who is behind the threatening letter before it's too late. The game plan? Use Kimihito as bait by sending him on a series of dates with monster girls. Along the way, several possible suspects emerge, each of whom has a "D" connection of one kind of another. Meanwhile, Kimihito's potential brides each begin acting strangely. Are they taking the fake dates too seriously--or is something (or someone) far more sinister at work?