Lala

"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, but they appear to be a Dullahan.

First "D"
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. In Chapter 22 it is revealed that "D" wrote another letter that was similar to a threatening love letter. In the end it was revealed that Doppel wrote the first letter to Ms. Smith's to help facilitate who Kurusu would pick to marry, after hearing Ms. Smith complain about it. However, she reveals that she only wrote the first letter, thus culminating in a second D to appear.

Second "D"
The second D appears to be a Dullahan, who is interested in preventing Kimihito from marrying anyone for yet unspecified reasons. The letter she wrote is a lot more poetic than the one Doppel wrote.

Trivia

 * Fans theorized that "D" may be, in fact, a stalker of Kimito.
 * Another peculiar thing is the wording of the first letter itself. For some reason, "D" simply says "Marry Someone" as opposed to "Marry a Monster Girl" or something with similar connotations. This suggests that "D" simply has a problem with Kimito getting married and perhaps isn't a radical opposed to Cross-Species marriage. This helps support fan speculation that "D" may be a stalker.
 * After it was revealed that Doppel only wrote the first letter, and not the second, more poetic one, lends credence to this theory.