D

"D" is the name given to the mysterious person who sent Kurusu Kimihito an ill-prepare death threat informing that he/she will kill Kimihito if he marries to one of the girls in his house.

So far, D's identity remains shrouded in mystery.

Plot
Sometime, Kimihito received an anonymous death threat, which said that if he were to marry one of the girls, he would die. It was signed merely with a "D". To bait out the one responsible, Kimihito and Ms. Smith went on a false date, with MON keeping an eye on them. When this was unsucessfull, Smith asked Kimihito's harem to go on dates with him, while protected by members of MON, in the hopes the writer would reveal him/herself.

Trivia

 * It is currently unknown whether D is a monster or a human. Though, it is speculated in-story that it may be a monster.