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.

D is a Dullahan who claims to be an agent of Death.

Plot
After revealing her self to Kimihito and the others following a search for her lost head, she reveals that her species serves as form of Grim Reaper for people, arriving as a prelude to death, and in fact is perhaps, serious, that Kimihito's eventual marriage to any of his guest may result in his subsequent death.

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.